[WIP] User can reset his RSS token

What does this MR do?

It allows the user to reset his RSS token.

Follow-up to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11647.

• Add section in the user's account section to reset the rss token
• Remove dead code related to creating an initial security token.

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

I removed a piece of code that wasn't working anymore. The mentioned

Why was this MR needed?

Since we introduced rss_tokens in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11647 we need to allow the user to reset the token.

This MR is sponsored by @siemens.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• 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?

https://gitlab.com/gitlab-org/gitlab-ce/issues/29810#note_28404385