Only allow users with credit to post on the main forums

[ChipWolf]

This should protect those boards from spammers.

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[MrSpike63]

https://boinc.berkeley.edu/trac/wiki/ProtectionFromSpam

forum.post_min_expavg_credit
forum.post_min_total_credit

These need to be manually set in the database. You could just set the minimum credit to 1.

[ChipWolf]

Yeah, we shouldn't manually set anything in the database, it needs to be in this repo somewhere. We could use <profile_min_credit> in the config.xml to prevent people without credit creating a profile, which you need to post on the forum. My only concern with this is, what if people need to use the forum for support?