Multi::API: Expose popularity, rating and votecount

http://vndb.org/t5121
Multi::API: Fix two 'Use of uninitialized value in multiplication'

Shouldn't affect behaviour in any way, just get rid of the warnings.
DB::Releases: Fix mixing AND/OR expressions in platform/media filters

Fixes http://vndb.org/t5136
Shorten the VNDB description on the homepage

The stuff about promoting users to register isn't really necessary.