As part of the WMF Matrix trial (T230531: Run Matrix trial using the modular.im-hosted instance) bridge Matrix together with Libera so that Libera users can see and talk to Matrix users and vice versa.

• evaluate available bridge implementations (actually matrix-appservice-irc seems to be the only mature one?)
• consider setting up the bridge on the Modular.im instance vs. using a public bridge
• make sure that the Matrix integrations menu offers our choice of bridge, not the default bridge (if we don't choose to use the default)
• test bridging, especially with Libera anti-spam restrictions (which has been a pain point in past attempts)