URI decode Page-Title header to preserve UTF-8 characters

What does this MR do?

The Page-Title header has to be ISO-8859-1, so URI encode the characters to they'll be valid ISO-8859-1 and the frontend decodes again to get proper UTF-8 again.

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

Why was this MR needed?

ApplicationController was trying to convert unavailable characters to ISO-8859-1.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [-] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#39179.