Add more custom validators

These custom validators allow us to DRY up our models a bit.

See merge request !1944

Fix 500 error when creating a merge request that removes a submodule

Fixes #3476 

See merge request !1989

Add explicit reference to rouge 1.10.1

We are relying on rouge 1.10.1 but nothing is referencing to this version and it is easy to lose this dependence. Previously, it was updated directly in Gemfile.lock which is not reliable way :)

https://gitlab.com/gitlab-org/gitlab-ce/commit/e20529cec4f6de0c2c1589da99abb8714ea1c1c1

`gollum-lib` has a dependence `rouge 1.9` which is old for us.

See merge request !1937

Fix random failing test - delete attachment

Make sure we wait for AJAX request to finish before end test and cleanup
database
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

cc @rspeicher @DouweM 

Fixes #3885 

See merge request !2007

Make sure we wait for AJAX request to finish before end test and cleanup
database
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Fix spec failure introduced by 9d03bc6f

See merge request !2006

Fix random failing test

Make sure we wait till page reloads after request was merged. Otherwise
we get request running which fails next test

Improvement for !1897
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !2004

Fix #3758: Serious performance issues due to timeago()
being called n*(n+1)/2 times instead of n

See bug #3758 for a description. This merge request alters 
`time_ago_with_tooltip` to invoke the `timeago()` javascript on the
current timestamp only, instead of each one defined on the page so far.

See merge request !1977

Dont use cached collection for Repository find_branch and find_tag methods

Fix for #3816 
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !2003

Make sure we wait till page reloads after request was merged. Otherwise
we get request running which fails next test
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Filter current user to top of assignee list in issues/merge requests

### What does this MR do?

Fixes #2076

Filter the current user to the top of the assignee list in issues and merge requests. This makes it fast/easy for a user to assign themselves without compromising the UI. This is the same thing we do in the filter select boxes in other places.

### Are there points in the code the reviewer needs to double check?

No.

### Why was this MR needed?

Users requested the feature in #2076



See merge request !1493