Force predefined list of networks?

**Is your feature request related to a problem? Please describe.**
Right now it is only possible to either force Convos to a single IRC network or allow connecting to any network. As many networks have strict connection limits from a single IP (without requesting special permissions) this poses a certain problem if you want to allow connections to multiple networks.

**Describe the solution you'd like**
It would be nice to have an option to only allow connections to those that have a predefined default configuration made by an admin. This way one can ensure WEBIRC etc. is used and avoid having your IP ban-listed on some not-configured IRC networks.

**Describe alternatives you've considered**
Of course forcing to a single network works, but it would be nicer if limited pre-configured connections to some other networks were possible.

No files, tests, or entry points are named. Start by tracing how the existing single-network restriction and admin-defined default configurations are loaded, then determine how a predefined allowed-network list should affect connection attempts; done means admins can permit several configured networks while preserving their connection settings.

Written by the indexing model from the issue text.