Consistent version in documentation URLs

This now uses the plugin manual helper to redirect users. It
consistently uses the same version as the REX documentation buttons.

Ideally it would also use the plugin_manual type in documentation_url,
but that doesn't work in Foreman itself.

app/helpers/remote_execution_helper.rb

@@ -203,8 +203,7 @@ def time_in_words_span(time)
   end
 
   def documentation_button_rex(section = '')
-    url = 'http://theforeman.org/plugins/foreman_remote_execution/' +
-      "#{ForemanRemoteExecution::VERSION.split('.').take(2).join('.')}/index.html#"
+    url = "https://theforeman.org/plugins/foreman_remote_execution/#{rex_doc_version}/index.html#"
     documentation_button section, :root_url => url
   end
 
@@ -276,4 +275,8 @@ def targeting_hosts(job_invocation, hosts)
         actions: template_invocation_actions(task, host, job_invocation, template_invocation) }
     end
   end
+
+  def rex_doc_version
+    ForemanRemoteExecution::VERSION.split('.').take(2).join('.')
+  end
 end

app/views/job_invocations/welcome.html.erb

@@ -7,7 +7,8 @@
         <%= _("Foreman can run arbitrary commands on remote hosts using different providers, such as SSH or Ansible. Communication goes through the Smart Proxy so Foreman does not have to have direct access to the target hosts and can scale to control many hosts.") %></br>
     </p>
     <p><%= link_to _('Learn more about this in the documentation.'),
-           documentation_url('1.ForemanRemoteExecution1.3Manual', :root_url => 'https://www.theforeman.org/plugins/foreman_remote_execution/1.3/index.html#'), :rel => 'external' %></p>
+      external_link_path(type: 'plugin_manual', name: 'foreman_remote_execution', version: rex_doc_version, section: "#1.ForemanRemoteExecution#{rex_doc_version}Manual"),
+      rel: 'external' %></p>
     <div class="blank-slate-pf-main-action">
         <%= display_link_if_authorized(_("Run Job"), { :action => :create }, { :class => "btn btn-primary btn-lg" }) %>
     </div>