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 e4b54c37fe,
we expect double-puppeting to require the new appservice double-puppeting method.
This commit switches the mautrix-slack bridge to it.
