Resolve several Rails/IncludeUrlHelper violations

What does this MR do and why?

Attempts to resolve a majority of issues collected in &6767 (closed). Part of gitlab-com/support/support-training#1982 (comment 730351904).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

I used RubyMine's Go To Tests & Run 'RSpec:…' with Coverage to check whether any of the lines I change here needs new tests, as in !71235 (diffs). I think I've found none, but I'd appreciate a 2nd confirmation of this, or different advice about how to check.