Fix front-end for branches that happen to contain urlencoding escape characters (e.g. %)

Originally opened at !3574 (closed) by @ewiltshi.

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Adding a branch with a name like "foo%20bar" (via command-line) and setting it to the project's default branch causes the project "show" page to 404 for that project. "assign_ref_vars" unescapes the branch name ("foo%20bar" ==> "foo bar"), making GitLab look for a non-existent branch. This MR adds logic to skip the URL unescaping step in "assign_ref_vars".

Fixes #10772 (closed), #14992 (closed), #15304 (closed).

Does this MR meet the acceptance criteria?

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