This reverts commit 13e37a3e.

Fixed issue when nav wasn't present

Closes #18542

Added sidebar
Removed elements not present in design

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Add fade out to left side of layout nav; remove dropdown nav helper; remove controls button from mobile

Shrinks when navigating to diff view.
Shrinks when refreshing on diff view.
Expandable at anytime.

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.

with new sidebar
Fixes #3981

[WIP]

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