Added us_status column for future expansion re Bryan's suggestion

Added us_status column for future expansion re Bryan's suggestion
Implemented changes suggested in code review on r92009:
 constructor bug handling passed repo/stash
 up-to-date timestamp generation
 fetching db handle from repo
 iterating over select results according to convention
 changed uploadstash.us_media_type to enum to mirror image.img_media_type
 removed (most) new references to $wgUser, instead using ApiBase::createContext to find the user