Fix link togglers jumping to top

What does this MR do?

• e.preventDefault() only for a and button. We want the checkbox to do its own thing.

Using <button> is semantically more accurate for the usage and we don't need the previous e.preventDefault()

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

Togglers to check:

• "Create a Mattermost team for this group" checkbox, http://localhost:3000/groups/new (enable mattermost in config/gitlab.yml)
• "Repo by Url" button, http://localhost:3000/projects/new
• In MR widget "Modify commit message", http://localhost:3000/gitlab-org/gitlab-ce/merge_requests/3
• Discussion "Toggle discussion", http://localhost:3000/gitlab-org/gitlab-ce/merge_requests/3
• "Showing 1 changed file with 4 additions and 7 deletions" , http://localhost:3000/gitlab-org/gitlab-ce/merge_requests/3/diffs
• Commit description toggle "...", http://localhost:3000/gitlab-org/gitlab-ce/commits/master

Why was this MR needed?

• We used a tags with empty fragment href="#" which would jump you to the top of the page when clicked

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • All builds are passing
• 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?

Closes #29414 (closed)