Branch rules edit: separate mounting modules

What does this MR do and why?

This is the last MR in the series of refactors (#435143 (closed)).

I'm moving apps outside of the components directory:

• group_settings
• project_settings
• mr_edit

I decided not to rename the js files to index.js nor to create components directory inside each app, to minimise the changes. This means that approvals/components directory will contain both shared and app specific components for now.

There's also been some specs that were not grouped into the right directories after I moved their respective components.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

There's no UI facing change, since it's only moving the files. However, here are the screenshots of the vue apps that are affected:

1. Project settings

2. Group settings

3. MR edit

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #435143 (closed)