Refactor edit_on_gitlab helper (again)

Merge !2861 (merged) first.

This is a re-do of !2845 (merged), which was reverted due to a bug.

The runner repository has documentation stored in docs instead of doc, so links to the runner project were going to an incorrect file path. This time, I've addressed this by adding a doc_directory attribute onto PRODUCT_REPOS. This accounts for repos having different names for their documentation directories.