Show loading indicator for autocomplete

Closes#15435
See merge request !4182
#17284 Edit form does not cancel correctly
## What does this MR do?
Replaces the markdown textarea with the currently saved note text when the note edit is cancelled.
## Are there points in the code the reviewer needs to double check?
No
## Why was this MR needed?
Allows the user to revert back to the original note text, as expected, using cancel.
## What are the relevant issue numbers?
Fixes#17284.
## Screenshots

See merge request !4175
Updated CHANGELOG
Corrected the changes to store the markdown in a hidden input when edit starts and retrieve the original from that input when edit is cancelled
Replaced hidden input with form attribute
Remove prev/next buttons on issues and merge requests
## What does this MR do?
Remove prev/next buttons on issues and merge requests sidebar
## Are there points in the code the reviewer needs to double check?
No
## Why was this MR needed?
The buttons were rarely used and added at least 1 query each on every page load which we could live without.
## What are the relevant issue numbers?
#18026
## Screenshots (if relevant)
Below is how it looks. Not sure if it's too much empty space. #17697 discusses a possible substitute for what to put there.


See merge request !4442
Notes are awardables
## What does this MR do?
Makes sure we can :thunder_cloud_rain: comments/notes.
## What are the relevant issue numbers?
Follows up upon !2901, depends on !3785Closes#3655
## Screenshots (if relevant)
TODO
See merge request !4291
Upgraded jQuery to version 2
## What does this MR do?
Upgrades jQuery to 2.2.1.
Had to include the task_list JS file directly as it includes jQuery 1 directly https://github.com/github-archive/task_list/blob/master/app/assets/javascripts/task_list.coffee#L8 so when we change jQuery to `jquery2` it was including both jQuery 1 & 2.
## Are there points in the code the reviewer needs to double check?
For any JS errors
## What are the relevant issue numbers?
Closes#12440
See merge request !4384
- Move the `TwoFactorAuthsController`'s `new` action to `show`, since
the page is not used to create a single "two factor auth" anymore. We
can have a single 2FA authenticator app, along with any number of U2F
devices, in any combination, so the page will be accessed after the
first "two factor auth" is created.
- Add the `u2f` javascript library, which provides an API to the
browser's U2F implementation.
- Add tests for the JS components