Remove limit of 600 private messages

[krzysiobal]

Got that warning today and now I have to decide which messages to remove and which to keep. Do we really need to have such limits?

600 messages, each of around 1kB (at most) makes 600kB of used database space (excluding attachments if any).
It is still less than just an average single forum attachment.

[Fiskbit]

I've increased this to 800 for now and will discuss with the rest of staff to see if we need a limit at all.

[Fiskbit]

I've created a new group for heavy users of private messages that has a much higher limit and reverted it for everyone else. If anyone runs into the limit again, please reach out.