Sidebar fixes
Fixes#13215
* when editing a dropdown, but not changing anything you end up in an ugly state
* the hover-over state for the subscribe and other buttons are veeery subtle
* the collapse button needs some sort of hover-over state
* the reference gets shortened. It always got shortened. Not an issue with this in particular.
* the minimized sidebar is not functional. The copy button doesn't do anything
* looks all funky in safari (not for me)
See merge request !2736
1. We now display the index of the current issuable among all its project's
issuables, of the same type and with the same state.
2. Also, refactored a bit the Issuable helpers into a new IssuablesHelper
module.
3. Added acceptance specs for the sidebar counter.
You can set type: :merge_request to create a link to merge request
lists instead of issues list. Accept both Symbol & String as type
argument for the link_to_label helper
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.
Fix headers of milestone and snippet show pages
Fix layout, and move "Edit" button to very right just like issue/MR.
Before:


After:


cc @skyruler
cc @rspeicher 8.3 please!
See merge request !2104
UI: Improve form consistency
Depends on !1953
See the commits for more details, the messages mostly speak for themselves.
# Highlights
## Tag form
Before:

After:

See merge request !1955