Transcript request feature

Implements requests for transcripts on videos, audios and series by modifying existing digitization feature.

Activeadmin shows type of digitization request so you can see if a request on an item is a transcript or basic digitization request. 

Improves the look of the request buttons.

Fixes the redirect link so user is sent to the right place after making requests and after viewing items in the digitization requests tab on their profile page.

Logic for displaying the transcript request button based on if a transcript already exists and whether or not the user has already requested a transcript.

*** note about the DISABLE STI line in Artifact.rb - Rails uses the column "type" for single table inheritance. I wanted to use this type column for something else - as in "type" of digitization or transcription, so I renamed the column Rails would use for STI just on the artifact model.