Unified mirror config page

[Bethibande]

Task

• Mirrors have become more and more complex, now the settings don't really fit in a single connection card anymore and I still want to add various new settings
• Users may also want to have multiple repositories that mirror the same target repository without copying all of the connection details
• Having a dedicated page for mirror settings may be a good idea
• Users should be able to create a new mirror, name it, specify a package manager and then the connection details, including a reference to an existing internal repository
• Then you can simply select a list of mirrors for your repository to include and be done with it
• This new page must also be accessible from within the repository settings, ideally it should not reset the current configuration changes