[feature/api] Small fix to check correct permission

Previously the forum repository checked incorrectly for the f_read
permission when listing forums. From now on it checks f_list instead.

PHPBB3-11595
[feature/api] Changed posting to use json instead of multiple post data

Having a post field for each field wasn't optimal due to hashing.
Developers would have to structure their arrays exactly as done in
phpbb. With json ordering doesn't matter as they will have the same
order on both sides when decoded.

PHPBB3-11595