Hide and show icons in collapse and expanded mode.

[WIP]

The pattern in the `::reference_pattern` class method in the
ExternalIssue model does not match all valid forms of JIRA project
names. I have updated the regex to match JIRA project names with numbers
and underscores. More information on valid JIRA project names can be
found here:
https://confluence.atlassian.com/jira/changing-the-project-key-format-192534.html

* The first character must be a letter,
* All letters used in the project key must be from the Modern Roman Alphabet and upper case, and
* Only letters, numbers or the underscore character can be used.

Uses `$border-color`

This reverts commit a1408948.

This reverts commit 11183488.

Fixes #12725

Introduced by me through !2005

New download btn WIP.

Fixes #2498

Fixes #2307

Only top search is gray.

Main search input has different styles than secondary search inputs.

All inputs of type `search` will have the gray background
and search icon centered.
Because the search magnifier is a bg image,
I had to hide it with `[value=""]`.
I added a little javascript to make sure each input always has it's
own value.

Closes #12672

Closes #12677

The route is supposed to redirect the Groups#index request based on
whether or not a user was logged in. If they are, we redirect them to
their groups dashboard; if they're not, we redirect them to the public
Explore page.

But due to overly aggressive `before_action`s that weren't excluding the
`index` action, the request always resulted in a 404, whether a user was
logged in or not.

Closes #12660

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

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

Leaving out the -I option in "git grep" would cause an Error 500 because the
resulting line would include "Binary file X matches"