Notes/discussions header

Part way towards getting the design for issues/merge requests to look more like the design in #3401 - I have standardised the headers for both issues & merge requests comments and discussions.

See merge request !3441

Added & use Gitlab::Routing for URL helpers

Extracted from !3389

See merge request !3486

[ci skip]
Signed-off-by: Rémy Coutable <remy@rymai.me>

Memoize reference_pattern/link_reference_pattern

Extracted from !3389

See merge request !3485

spec and fix for fogbugz lonely user problem

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/14766

I encountered this issue while manually testing all import types for
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3066

This is really due to a horrible API

```
{ 'people' => { 'person' => array_of_people_or_single_person_as_hash } }
```

See merge request !3457

Revert "Merge branch 'users_should_not_be_able_upvote_downvote' into 'master'"

This reverts merge request !3406

See merge request !3491

Allow temp-oauth-email as notification email

Closes #14559 

/cc @dblessing @rymai 

See merge request !3477

This reverts merge request !3406

Prevent users from being able to both upvote and downvote

closes #14480 

![thumbs](/uploads/d0d64f08dbf271fe969b44c68f424123/thumbs.gif)

See merge request !3406

Fix divider in dropdown when not needed

closes #14533
## Before
![Screen_Shot_2016-03-25_at_12.35.21_AM](/uploads/20f386c9c45c9ee8716835ccfa0066ed/Screen_Shot_2016-03-25_at_12.35.21_AM.png)

## After
![Screen_Shot_2016-03-25_at_12.37.27_AM](/uploads/22db62db920017da5fe1da7670fbbb0e/Screen_Shot_2016-03-25_at_12.37.27_AM.png)

See merge request !3394

Rails' "url_helpers" method creates an anonymous Module (which a bunch
of methods) on every call. By caching the output of this method in a
dedicated method we can shave off about 10 seconds of loading time for
an issue with around 200 comments.

These methods are called quite often in loops so by memoizing their
output we can reduce timings a bit.

Fixes issue with dropdowns not selecting values

Fixes issue with dropdowns not selecting when navigating between tabs

Closes #14813

See merge request !3478

Merge branch 'snippets-with-comments-cause-a-500-when-they-show-up-in-search-results-14764' into 'master'

Fix Error 500 when searching for a comment in a project snippet

Closes #14764. /cc @stanhu, and thank you for the spec! ;)

See merge request !3468

Mentions on confidential issues doesn't create todos for non-members

Closes #14569 

See merge request !3374

Rewrite uploads when moving issue to another project

Closes #14531

See merge request !3382

Remove "Congratulations!" tweet button on newly-created project.

I’ve removed everything related to the feature based on this commit: ce08f919

Resolves #10857.

See merge request !3451

I’ve removed everything related to the feature based on this commit:
ce08f919f34fd8849834365

Resolves #10857.

Minor clean up on admin/users_controller_spec

Extracted from !3439 

See merge request !3460

Fix input borders



See merge request !3474

Fixes issue with dropdowns not selecting when navigating between tabs

Closes #14813

Exclude projects pending deletion from all results

Closes #14138

See merge request !3439

Destroy related todos when an Issue/MR is deleted

Closes #14550 

Closes #14598

See merge request !3376

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

See merge request !3472

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

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

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