Added server_seconds_paid field to /user/{username}/about.json api

Contains the value as a float, or null if the user does not wish to
disclose the information.
Resolves #1045.
The issue with moving the code into gold.py is that it makes use of Bid from bidding.py, which imports Link from link.py, which in turn import stuff from gold.py. 
However, it turns out bidding.py does not make use of Link at all. Deleting the import and changing the line "from r2.models import FrontPage" to "from r2.models.subreddit import FrontPage" in bidding.py makes it all work on my local instance. Is there a reason why it didn't specify subreddit in the import?
Also I've added a "server_seconds_gifts" attribute in the json template for server time that was paid on behalf of the user, rather than by the user themselves.