use the fallback if a user-interactive-auth type is unknown

[richvdh]

Currently, if you try to do an operation that requires UI Auth, and the server offers only flows with unsupported login stages, riot shows a "no supported flows" type of error.

Instead, I think that riot should pop open a new window or iframe or something, and use the fallback mechanism.

In combination with support from the homeserver (see matrix-org/synapse#5667), this could be used to fix #12028.