Show issuable context labels as blocks

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/projects/issues/_issue_context.html.haml

 = form_for [@project, @issue], remote: true, html: {class: 'edit-issue inline-update'} do |f|
   %div.prepend-top-20
-    %strong
+    %p
       Assignee:
     - if can?(current_user, :modify_issue, @issue)
@@ -11,7 +11,7 @@
       None
   %div.prepend-top-20
-    %strong
+    %p
       Milestone:
     - if can?(current_user, :modify_issue, @issue)
       = f.select(:milestone_id, milestone_options(@issue), { include_blank: "Select milestone" }, {class: 'select2 select2-compact js-select2 js-milestone'})

app/views/projects/merge_requests/show/_context.html.haml

 = form_for [@project, @merge_request], remote: true, html: {class: 'edit-merge_request inline-update'} do |f|
   %div.prepend-top-20
-    %strong
+    %p
       Assignee:
     - if can?(current_user, :modify_merge_request, @merge_request)
@@ -11,7 +11,7 @@
       None
   %div.prepend-top-20
-    %strong
+    %p
       Milestone:
     - if can?(current_user, :modify_merge_request, @merge_request)
       = f.select(:milestone_id, milestone_options(@merge_request), { include_blank: "Select milestone" }, {class: 'select2 select2-compact js-select2 js-milestone'})