Introduce "what next instructions" to submission API response

In clients that support it, the "what next instructions" will be
displayed after a new iteration is submitted.

This allows us to modify the message anytime, without having to wait for
users to update their cli version.

api/v1/routes/core.rb

@@ -45,6 +45,10 @@ def current_user
         def find_user
           User.where(key: params[:key]).first if params[:key]
         end
+
+        def site_root
+          request.host_with_port + '/'
+        end
       end
     end
   end

api/v1/routes/iterations.rb

@@ -115,6 +115,7 @@ class Iterations < Core
         locals = {
           submission: attempt.submission,
           domain: request.url.gsub(/#{request.path}$/, ""),
+          what_next_instructions: what_next_instructions(attempt)
         }
         pg :attempt, locals: locals
       end
@@ -135,6 +136,28 @@ class Iterations < Core
 
         pg :iterations, locals: { submissions: submissions }
       end
+
+      private
+
+      def what_next_instructions(attempt)
+        return '' unless $flipper['cli-call-to-action'].enabled?(attempt.user)
+
+        track    = attempt.iteration.track_id
+        exercise = attempt.iteration.slug
+        url = File.join('http://', site_root, 'tracks', track, 'exercises', exercise)
+        <<~INSTRUCTIONS
+          Your #{track} solution for #{exercise} has been submitted.
+
+          Programmers generally spend far more time reading code than writing it.
+          To benefit the most from this exercise, find 3 or more submissions that you can
+          learn something from, have questions about, or have suggestions for.
+          Post your thoughts and questions in the comments, and start a discussion.
+          Consider revising your solution to incorporate what you learn.
+
+          Yours and others' solutions to this problem:
+          #{url}
+        INSTRUCTIONS
+      end
     end
   end
 end

api/v1/views/attempt.pg

@@ -4,6 +4,7 @@ node language: submission.problem.language
 node slug: submission.slug
 node name: submission.problem.name
 node iteration: submission.version
+node what_next_instructions: what_next_instructions
 
 # deprecated
 node id: submission.key

test/api/assignments_test.rb

@@ -12,6 +12,7 @@ def app
   attr_reader :alice
   def setup
     super
+    $flipper['cli-call-to-action'].enable
     @alice = User.create(username: 'alice', github_id: 1)
     Language.instance_variable_set(:"@by_track_id", "fake" => "Fake")
   end

test/fixtures/approvals/api_multifile_submission_accepted.approved.json

@@ -5,6 +5,7 @@
   "slug": "one",
   "name": "One",
   "iteration": 1,
+  "what_next_instructions": "Your fake solution for one has been submitted.\n\nProgrammers generally spend far more time reading code than writing it.\nTo benefit the most from this exercise, find 3 or more submissions that you can\nlearn something from, have questions about, or have suggestions for.\nPost your thoughts and questions in the comments, and start a discussion.\nConsider revising your solution to incorporate what you learn.\n\nYours and others' solutions to this problem:\nhttp://example.org/tracks/fake/exercises/one\n",
   "id": "<id>",
   "status": "saved",
   "track": "fake",

test/fixtures/approvals/api_submission_accepted.approved.json

@@ -5,6 +5,7 @@
   "slug": "one",
   "name": "One",
   "iteration": 1,
+  "what_next_instructions": "Your fake solution for one has been submitted.\n\nProgrammers generally spend far more time reading code than writing it.\nTo benefit the most from this exercise, find 3 or more submissions that you can\nlearn something from, have questions about, or have suggestions for.\nPost your thoughts and questions in the comments, and start a discussion.\nConsider revising your solution to incorporate what you learn.\n\nYours and others' solutions to this problem:\nhttp://example.org/tracks/fake/exercises/one\n",
   "id": "<id>",
   "status": "saved",
   "track": "fake",

test/fixtures/approvals/api_submission_accepted_on_completed.approved.json

@@ -5,6 +5,7 @@
   "slug": "one",
   "name": "One",
   "iteration": 1,
+  "what_next_instructions": "Your fake solution for one has been submitted.\n\nProgrammers generally spend far more time reading code than writing it.\nTo benefit the most from this exercise, find 3 or more submissions that you can\nlearn something from, have questions about, or have suggestions for.\nPost your thoughts and questions in the comments, and start a discussion.\nConsider revising your solution to incorporate what you learn.\n\nYours and others' solutions to this problem:\nhttp://example.org/tracks/fake/exercises/one\n",
   "id": "<id>",
   "status": "saved",
   "track": "fake",