Related Issues UI Polish

What does this MR do?

• Use ... for badge number before we actually know a number
• Focus input on form expansion
• Enable submit button when text is in input
• Update generic submission error message: "We can't find an issue that matches what you are looking for"
• Get rid of "Add an issue" tooltip on badge
• Get rid of "Remove issue" x icon tooltip on badge
• Update submit button text to simply "Add"
• Change placeholder to "Paste issue link or <#issue id>"

https://gitlab.com/gitlab-org/gitlab-ee/issues/2756

Are there points in the code the reviewer needs to double check?

Each fix is in it's own commit.

Why was this MR needed?

Follow-up UI polish discussed in call, https://gitlab.com/gitlab-org/gitlab-ee/issues/2756

Does this MR meet the acceptance criteria?

• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2756 (closed)