Enable adding oneself to the roster.
Self subscriptions are implicit according to XEP-0060 / XEP-0163.
So we adapt the /sub command to handle this gracefully by just printing
an informative message.
Profanitys logic didn't handle own presence/when adding to roster
correctly. This got fixed now.
Fixes: https://github.com/profanity-im/profanity/issues/2084
Signed-off-by: Michael Vetter <jubalh@iodoru.org>
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