Migrate coq/opam to use Inria GitLab instead of GitLab.com. (#324)

Following an issue with having hit our quotas on GitLab.com, and by
request of @mattam82 in
https://coq.zulipchat.com/#narrow/stream/237656-Coq-devs-.26-plugin-devs/topic/GitLab.2Ecom.20subscription.20.2F.20compute.20minutes.

This PR does the following things:

- [x] Set up mirroring.
- [x] Modify configuration for synchronizing PRs.

We should also:

- [x] Create the Inria GitLab repository.
- [x] Adjust the GitLab CI configuration to use the shared runners
available there. (Done in coq/opam#3181.)
- [x] Add a webhook on the Inria GitLab repository.

coqbot-config.toml

@@ -24,9 +24,10 @@ app_id="31373"
   gitlab="coq/coq"
   gitlab_domain="gitlab.inria.fr"
 
-  [mappings.opam-coq-archive]
-  github="coq/opam-coq-archive"
-  gitlab="coq/opam-coq-archive"
+  [mappings.opam]
+  github="coq/opam"
+  gitlab="coq/opam"
+  gitlab_domain="gitlab.inria.fr"
 
   [mappings.math-comp]
   github="math-comp/math-comp"

src/bot.ml

@@ -288,7 +288,7 @@ let callback _conn req body =
                     branch on GitLab."
                    owner repo )
               ()
-        | "math-comp", ("docker-mathcomp" | "math-comp") ->
+        | "math-comp", ("docker-mathcomp" | "math-comp") | "coq", "opam" ->
             (fun () ->
               init_git_bare_repository ~bot_info
               >>= fun () ->