Enable Merge Request Resolve Discussion button on all related discussions

Description

As requested by customer:

Since Merge Requests are intended to be used to review work before it is committed into a branch, it would be useful to have a Resolve Discussion button show up for any non-change related discussions as well. Non-change related discussions can be necessary if a file was inadvertently omitted from the merge request (i.e. never committed/pushed) or it is a file that GitLab is not capable of diff'ing such as a Word doc or image file. The Settings option to "Only allow merge requests to be merged if all discussions are resolved" is very useful, but if there are Merge Request review comments that are unrelated to changes, currently they can slip through the cracks unresolved.

Proposal

A settings toggle for "Resolve discussion" button to appear on comments which don't depend on file changes.

Links / references

ZD: https://gitlab.zendesk.com/agent/tickets/72468

Documentation blurb

1. Required: because not all files can be "diffed" and sometimes files are omitted from the commit.
2. Solution: A settings toggle for "Resolve discussion" button to appear on comments which don't depend on file changes.
3. How: Customer enables the feature under settings.