Create a new backend endpoint to receive a dashboard.json file and return a commit/branch

As part of #27835 (closed)

The user should be able to modify the metrics_dashboard json content returned from <project-name>/environments/xx/metrics_dashboard.json and receive a new yml file which describes the dashboard with all the changes done.

Input:

• JSON payload which corresponds to the new dashboard configuration.
• (optional) Commit message
• (optional) Branch name
• (bool) if branch is sent, start merge request
• filename or dashboard identifier

Output:

• URL to merge request
• or, if commiting to master, URL to refresh the dashboard (?)

Corresponding form which will POST this info:

Technical notes

• This API provides an interface to stage and commit files: https://docs.gitlab.com/ee/api/repository_files.html#repository-files-api