refactor models
add dependency for YARD
doc: add YARD doc for Account model
doc: add YARD doc for Favorite and Retweet model
doc: add YARD doc for Notification class
doc: add YARD doc for Tweet model class
web: fix sessions#create
api: remove deprecated API endpoints
web: (security issue) fix unexpected open redirector on sessions#create
web: merge tweets#*_recent into tweets#*_best