Tag input UX improvements

Closes #4105

- Turns the tag icon white when input is focused.
- Focuses on the tag inout after deleting a tag.

Credit to @novaugust for a PR to this which is rebased into one single commit
Implement popovers
Merge pull request #4287 from PaulAdamDavis/tag-input-ux

Tag input UX improvements
Merge pull request #4213 from PaulAdamDavis/new-popovers

Implement new popovers
Move unrelated editor cover code to the right place

Closes #4106

Moved the `.editor-close` styles to `post-settings-menu.scss`