* mautrix-meta: enable spaces; add a hint into the display name

* use the meta mode to determine displayname suffix

* Allow for people to easily unset the mautrix-meta displayname suffix

Previously, unsetting `matrix_mautrix_meta_messenger_bridge_displayname_suffix`
or (`matrix_mautrix_meta_instagram_bridge_displayname_suffix`) variable would
make you end up witha trailing space in `displayname`.

It's possible that mautrix-meta trims this, but I haven't checked. It's
better not to risk it anyway.

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Co-authored-by: Slavi Pantaleev <slavi@devture.com>