Complete data scheme overhaul

 - individual tags instead of concatenated ones for each picture
 - Tags are associated to the tagging user.
 - Votes are associated to the voting user.
 - Comments can be voted up or down.
 - Images, comments, and tags can be marked deleted with a reason.
 - Prepare for password hashs produced by password_hash().
 - Store access tokens and image hashs as binary strings.
 - Store timestamps as BIGINT because other possibilities of MySQL
   and MariaDB suck or offer no real advantage over BIGINT.
 - Legacy user score and image ratings can be retained.
 - Lots of views do demonstrate and/or facilitate access to aggregated
   data
 - Abuse notification managment for things like spam, porn, copyright
   violations, or generally unethical or illegal content

I also included a crude migration script for old data as seen on Lowbird.
Support comment threads
Favorite images for users
Timestamp for image ratings
SQL: Force index in plv_images

MariaDB 5.5 doesn't pick up index for pl_imageratings by itself
Migrate database interaction library to PDO and prepared statements
Migrate main page to new data scheme

 * Refactor grid view computation:
   - Operate on copies instead of references and return the result,
     because copies are usually faster in PHP and make for a better
	 API.
   - Preserve original thumbnail list instead of sorting it back to
     that state.

 * MariaDB creates temporary tables and is slow when joining views.
   It is signifantly faster to incorporate the query of the view into
   the main query.
Introduce checks to prevent runtime warning
CSS: Fixed grid size for missing thumbnails
Remove some white space on line endings
My development configuration

Commit it, so I don't accidentally overwrite it again.

No, I don't use that password anywhere else, suckers!
Update DB class documentation
Don't show deleted images
lib/db.php: Facilitate other fetch modes
Migrate login to new DB scheme and passwords

 - Delete lots of obsolete methods.
 - Refactor large spaghetti methods into multiple smaller ones.