Added check for incorrect entry of youtube URLs as raw URLs in the video entry form. Closes issue #34
Moved video types constants into their own file to resolve circular import issue
Refactored the storage of videos. Videos are now stored in their own table
Added sqlite db to gitignore
Updated all other video containing classes to use the VideoAsset as a foreign key via django model inheritance
Added check for the same start and end time into the video entry form