Merge branch 'capitalize-mentioned-issue-notes' into 'master'

Capitalize mentioned issue timeline notes

## What does this MR do?
Renames `mentioned in` to `Mentioned in` for issue timeline notes

## Are there points in the code the reviewer needs to double check?
Shouldn't be any :smile: 

## Why was this MR needed?
Resolves existing UI inconsistency as all the other issue timeline notes have their first letter capitalized.

## What are the relevant issue numbers?
Closes #21416 

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-25_at_10.52.47_AM](/uploads/61de0a7d65fd739a6e565f6e63d2f52d/Screen_Shot_2016-08-25_at_10.52.47_AM.png)

After:
![Screen_Shot_2016-08-25_at_10.52.05_AM](/uploads/f67f353c285453306d6b80578b11e587/Screen_Shot_2016-08-25_at_10.52.05_AM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !6028

1 files changed, 5 insertions, 5 deletions

diff --git a/app/services/system_note_service.rb b/app/services/system_note_service.rb
index 546a8f11330..0c8446e7c3d 100644
--- a/app/services/system_note_service.rb
+++ b/app/services/system_note_service.rb
@@ -269,11 +269,11 @@ module SystemNoteService
   #
   # Example Note text:
   #
-  #   "mentioned in #1"
+  #   "Mentioned in #1"
   #
-  #   "mentioned in !2"
+  #   "Mentioned in !2"
   #
-  #   "mentioned in 54f7727c"
+  #   "Mentioned in 54f7727c"
   #
   # See cross_reference_note_content.
   #
@@ -308,7 +308,7 @@ module SystemNoteService
 
   # Check if a cross-reference is disallowed
   #
-  # This method prevents adding a "mentioned in !1" note on every single commit
+  # This method prevents adding a "Mentioned in !1" note on every single commit
   # in a merge request. Additionally, it prevents the creation of references to
   # external issues (which would fail).
   #
@@ -417,7 +417,7 @@ module SystemNoteService
   end
 
   def cross_reference_note_prefix
-    'mentioned in '
+    'Mentioned in '
   end
 
   def cross_reference_note_content(gfm_reference)