Merge branch 'patch/geo-mergerequest-500' into 'master'

Geo: Merge requests on Secondary should not check mergeable status

In a secondary Geo (#76) node, database is read-only. 

If you update your merge request and don't visit the primary node, you would end up with an unchecked `merge_status`.
We should not try to solve that in a secondary node, as this will lead to a 500 error because we can't update data on database.

This MR Fixes that:

```
Started GET "/root/test-project/merge_requests/1" for 189.101.232.69 at 2016-04-28 11:37:28 -0400
Processing by Projects::MergeRequestsController#show as HTML
  Parameters: {"namespace_id"=>"root", "project_id"=>"test-project", "id"=>"1"}
Completed 500 Internal Server Error in 372ms (ActiveRecord: 26.9ms)

ActionView::Template::Error (PG::ReadOnlySqlTransaction: ERROR:  cannot execute UPDATE in a read-only transaction
: UPDATE "merge_requests" SET "merge_status" = $1 WHERE "merge_requests"."id" = $2):
    39:     = render "projects/merge_requests/show/how_to_merge"
    40:     = render "projects/merge_requests/widget/show.html.haml"
    41:
    42:     - if @merge_request.source_branch_exists? && @merge_request.mergeable? && @merge_request.can_be_merged_by?(current_user)
    43:       .light.prepend-top-default
    44:         You can also accept this merge request manually using the
    45:         = succeed '.' do
  app/models/merge_request.rb:122:in `call'
  app/models/merge_request.rb:122:in `block (2 levels) in <class:MergeRequest>'
  app/models/merge_request.rb:272:in `check_if_can_be_merged'
  app/models/merge_request.rb:297:in `mergeable?'
  app/views/projects/merge_requests/_show.html.haml:42:in `_app_views_projects_merge_requests__show_html_haml__3632812159035673711_50958700'
  app/views/projects/merge_requests/show.html.haml:1:in `_app_views_projects_merge_requests_show_html_haml___4126596802437062695_51012840'
```

See merge request !366

app/models/merge_request.rb

@@ -258,7 +258,7 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def check_if_can_be_merged
-    return unless unchecked?
+    return unless unchecked? && !Gitlab::Geo.secondary?
 
     can_be_merged =
       !broken? && project.repository.can_be_merged?(source_sha, target_branch)

app/views/projects/merge_requests/widget/_open.html.haml

 .mr-state-widget
   = render 'projects/merge_requests/widget/heading'
   .mr-widget-body
-    - if @project.archived?
+    - if Gitlab::Geo.secondary?
+      = render 'projects/merge_requests/widget/open/geo'
+    - elsif @project.archived?
       = render 'projects/merge_requests/widget/open/archived'
     - elsif @merge_request.commits.blank?
       = render 'projects/merge_requests/widget/open/nothing'

app/views/projects/merge_requests/widget/open/_geo.html.haml

+%h4
+  = icon('exclamation-triangle')
+  Merge requests are read-only in a secondary Geo node