Make repository URL depend on remote storage

At the moment, when a repository is created, the user can provide a repository URL.

However, this is not good UI since many users don't know where their repository will be stored, let alone the exact URL for it.

So the following changes are proposed:

• remove URL form field from repo creation form
• set repository URL to the URL of the first storage being added
• when adding another remote storage, provide a checkbox to declare this storage, the main one
• when this checkbox is checked, the repo's URL will be replaced by the one of the storage
• when a remote storage is being updated and it was the main one, also update the repo URL, the QR code, the page, etc.
• only one repo's remote storage can be the main one for that repo