Improve performance of viewing individual issues

This MR does two things:

1. `Issue#related_branches` no longer performs Git operations that aren't needed
2. The output of `Repository#exists?` is now cached and flushed properly

Combined these two changes should further cut down the amount of Git operations performed when viewing individual issues (and possibly other pages).

See merge request !3296

These tests would check if the "This project is empty" banner would
contain SSH clone URLs. Oddly enough this should have never passed (as
far as I can tell) as SSH clone URLs in this banner are _only_ displayed
if the current user has at least 1 SSH key attached. Since the tests
never seem to create any they never should have passed, yet somehow they
did. To solve this the Spinach tests in question now ensure at least 1
SSH key is present.

Similar to ad90dba5185e30883d5ad6008c166b0df0108ebf we always have a
repository in the real world, so let's also create one in our Spinach
tests.

Fixes issue with close button not working on MR

Closes #14383

See merge request !3295

Add confidential issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/3678

More information: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/227

See merge request !3282

Adds small AJAX optimistic functionality to todos.

Fixes #13656 <br/>
A good first step and boring solution. <br/>
Will make ajax call to remove each issue.  <br/>
If issue is last in group of issues will refresh page.  <br/>
If issues remain in group will remove row with JS.  <br/>
Adds loading spinner to button and disables. <br/>

![todos-optimistic](/uploads/c6aec4a688e5125d4df55a2a3a55d4d7/todos-optimistic.gif) <br/>

cc @dzaporozhets @JobV @dbalexandre 

See merge request !2946

before the search field is focused