Merge remote-tracking branch 'upstream/master' into use-string-in-describe

* upstream/master:
  Expire the branch cache after `git gc` runs
  Be explicit on merge request discussion variables
  Improve wording in UI guide
  Fix expandable diffs
  CHANGELOG was updated.
  Style of import project buttons were fixed in the new project page.
  api: expose {should,force}_remove_source_branch
  Fix typo in UI guide
  Add rule about adding new header tab to the ui guide
  Update the health_check gem to the latest release
  Display tooltip for "Copy to Clipboard" button (!5164)
  Remove icons from some buttons which already has text