* Add matrix_bridges_msc4190_enabled flag for using msc4190 on supported mautrix bridges.
* Apply to_json to msc4190 in mautrix configs
* Add | to_json to mautrix bridge registration io.element.msc4190.
* require matrix_synapse_experimental_features_msc3202_device_masquerading_enabled for matrix_bridges_msc4190_enabled
* Also add msc4190 support for mautrix-telegram
We'be already been going against upstream defaults and have been
enabling backfilling for a few other bridges (Messenger, Instagram, Telegram, Twitter).
Now I'm enabling backfilling by default for the remaining ones, for
consistency.
While working on upgrading the Meta bridges to bridgev2, I've noticed
that {% raw %} and {% endraw %} on lines like that (immediately
preceding `username_template` may cause YAML indentation issues.
Since upgrading mautrix-slack (and pinning to v0.1.0) in e4b54c37fe258b17f49dd7ed58a18ef3abac1c41,
we expect double-puppeting to require the new appservice double-puppeting method.
This commit switches the mautrix-slack bridge to it.
