Fix token deletion problem when at end of line.

What does this MR do?

Fixes issue mentioned here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15348#note_53196232

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

Check that the tokens are immediately delete-able when:

1. no tokens are currently populated in the input.
2. tokens are present in the input.
3. using only keyboard to input tokens.
4. using only mouse to input tokens.

Why was this MR needed?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15348#note_53196232

Screenshots (if relevant)

None. Try it.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes: https://gitlab.com/gitlab-org/gitlab-ce/issues/41628