Some documentation on how to write database migrations.

This commit sponsored by Andrei Cristian Petcu.  Thanks!
More clarifications on what ./bin/gmg dbupdate does on each run.

This commit sponsored by Jessica Howard.  Thanks, sistah!
added user upload limits
added tests
typos
made no upload limit the default
fixed tests and defaults
fixed tests and defaults
fixed tests
max file size
fixed tests and defaults
added tests
copying.txt
duplicate
fix after rebase
javascript limit validation
use media.get_uploader and fix typo