disable federation if sqlite is used

[matrixbot]

This issue has been migrated from #2917.

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unless the admin set a yes_i_want_my_server_to_catch_on_fire option. This preserves the ability to get something up and running for testing, but reduce the surprise of peoples' servers melting when joining a large federated room like HQ. Of course, there will need to be a big warning somewhere so that people know that federation is disabled.