Fix underscore template error.
Fix `=` spacing

Milestones needed a none selector
Assignee needed unassign.

Added nice animations too.

Make add single and add multi work properly.
Multiple inputs for multiselect

Adds deselect still a WIP.

Also fixed alignment bug with can't merge badge

Fixed button hover color

Updated the spacing throughout the sidebar so that it fits better on smaller screens
Added more participants button

Closes #13353

1. Make the "subscribed" text in Issuable sidebar reflect the labels
   subscription status

2. Current user mut be logged-in to toggle issue/MR/label subscription

1. Allow subscribing (the current user) to a label

- Refactor the `Subscription` coffeescript class
  - The main change is that it accepts a container, and conducts all
    DOM queries within its scope. We need this because the labels
    page has multiple instances of `Subscription` on the same page.

2. Creating an issue or MR with labels notifies users subscribed to those labels

- Label `has_many` subscribers through subscriptions.

3. Adding a label to an issue or MR notifies users subscribed to those labels

- This only applies to subscribers of the label that has just been
  added, not all labels for the issue.

Closes #14195

Closes #13764

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.