Resolve "Cannot filter issues by labels ending with a plus char in board view"

What does this MR do?

Allow encoded + symbol in search filters. Fixes filters for any labels/milestones that have + symbol in them.

This also happens in: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/javascripts/filtered_search/filtered_search_manager.js#L466

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

This should maybe be de-duped

Why was this MR needed?

~bug

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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 #41715 (closed) Closes #34252 (closed)