Recognize admin commands without : after tag

Currently, only admin commands that have a colon directly after the @conduit:server_name tag are recognised as admin commands. This is useful because Element inserts the : automatically. However, many other Matrix clients (e.g. FluffyChat) don't insert this colon and you have to insert it manually for each admin command.

This MR fixes this by also recognising tags without a following colon as admin commands.

It wouldn't be good to just shorten the expected string to "{server_user}", since then $(@conduit:server_name).com would also be recognized as an admin command.

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• I agree to release my code and all other changes of this MR under the Apache-2.0 license