Multi-file management in Pipeline Editor

The pipeline config file list should allow user to:

Add any files related to the pipeline configuration to the Pipeline Editor page
Remove the files that are no longer part of your configuration (it only removes the file from the page, doesn't delete the file).

Envisioned user experience

Someone in charge of devops or at least to get the pipeline going, open our section and starts working
They add every CI config files they care about to their favorite, plus perhaps a few script files
Any other developer that need to edit these files now see every CI config file available as favorite and can open them
If any file is missing from the favorites that they need, they can either just open it manually every time or if it's a recurrent pain, they can someone with permission to add it
If there are too few people who can add it, add more people who can (groups)
If there are too many people and the list is not well maintain, remove people from group
If someone different add a new CI config file, but doesn't add it to favorite (either forgets or doesn't have the right) then if this file is often edited, then whoever is in charge can add it to the favorite (it's still accessible with full path though) or decide it's not used enough.

Requirements

User with necessary permissions can link any files as related to their configuration in the Pipeline Editor to make them easy to access. These files once added to Pipeline Editor become accessible across the project. Clicking on any of the linked files opens that file in the Pipeline Editor.
If user pastes a broken link when linking a config file, we show a validation error like we here.
We should show a tooltip when hovering over the "?" in the files section.
Once there's at least one config file, we should make it possible to collapse and expand the files section.

Edited Jan 20, 2021 by Nadia Sotnikova