Expand attribution for github files

[charlesreid1]

it would be nice to be able to attribute github files to users appropriately (as well as to add some datetime info).

Search schema changes:

• make search schema index github_user as a whoosh.fields.TEXT type

Github files re-indexing changes:

• extract a list of all users who have edited a particular document
• store a list of github users in the github_user field in the format "user1, user2, user3"
• extract the datetime of the first commit where the file appeared
• extract the datetime of the latest commit where the file appeared

(to do last two items, we need to fire up an API instance and iterate through files, and see what information is available and how.)

Github issues re-indexing changes:

• don't just attribute the person opening the issue to the github_user field, also include users who have commented and users who are assigned the task

Once everything is finished:

• re-index everything with beta
• copy over search index from beta to live