Remove icons from some buttons which already has text

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/18324


* remove plus icon from new milestone button on group milestone page
* remove plus icon from new project button on group projects page
* remove plus icon from milestone page

See merge request !5050

Expire the branch cache after `git gc` runs

Due to a stale NFS cache, it's possible that a branch lookup fails while `git gc` is running and causes missing branches in merge requests.

I'm not totally convinced this is the right solution, but since we and our customers are experiencing this issue quite frequently, I'm taking a stab at it.

Possible workaround for #15392


See merge request !5160

Be explicit on merge request discussion variables

## What does this MR do?

To avoid conditionals and to messing with request.format and accept headers to know in which format we're going to response I've decided to be explicit in when we need the discussion variables

## Why was this MR needed?

Solve a bug https://sentry.gitlap.com/gitlab/staginggitlabcom/issues/8492/

## What are the relevant issue numbers?

Closes #19702 

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- ~~[ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- ~~[ ] API support added~~
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5204