Fixed privilege checks for changing avatars
Added transaction manager
Moved functions to outer scope
Reduced post upload presenter complexity somewhat
Split EntityConverter to separate strategies

Reflection turned out to be bad, since I cannot implement my own method
in entities, like Post::setUser() instead of Post::setUserId().
Moved base64 decoding to FormData
Moved type conversion to EntityConverters
Turned errors into exceptions
Implemented post uploads (closed #11)
Bypassed phpcs warnings that shouldn't be there
Added checks for malformed JSON data
Minified exception traces