Fix people accidentally invoking OCRbot

[Lynnesbian]

One possible solution would be to ignore any mentions that don't come first - for example, respond to
@ OCRbot @ userA
but ignore
@ userA @ OCRbot

This solves the problem of people not realising why OCRbot ignored them when mentioned, but also means that more than one person can ask OCRbot to caption the same post.

[mahlzahn]

If userA starts an answer to an OCRbot message, then by default the answer starts with

@ OCRbot @ userB

This happened e.g. here: https://social.dsmouse.net/@MrsMouse/109423777972235807

I see two options:

1. Prevent OCRbot from processing his own posts (maybe that should be easy to implement).
2. Send error messages not to thread but as private messages to the users invoking the OCRbot, but this may raise questions to new users where others in the thread could help.