test grader caching (#2050)

Generalizes the failing fake autograder, new mode where it will succeed
n time and then fail.
cache interactive autograder results (fix #2050)

Use our simple filesystem-based cache to store results from the remote
grader.  We only cache for two hours for now, which should still be
useful, but we could consider going longer (days).

The cache key is the entire params structure that includes both the
student submission and the grader parameters (grader name, question
number).  This errs on the side of safety.

Some imports and variable cleanups while I was in here.
remove errant debug stmt (#2050)
added twitter block to class homepage, but it needs to be generalized...
changed to using 'course_prefix' as part of the twitter handle (eg. @ 'db' class) this should pretty much fix #2247
changing back to hardcoded twitter handle, will revisit later with input from @jinpa and others (#2247)
moved code into a django block and made it appear only on the homepage
grader caching: chef recipe to create db table (#2050)
grader caching: create DB table with every app server (#2050)

This is overkill, since we would only really ever need to do this once,
and then from only one app server.  But for now this is fine since it
fails quickly and is idempotent.
grader cache: switch to db-based cache (#2050)

Expect this to improve hit rates across multiple app servers.  This is a
special operation that we're caching -- infrequent and very expensive.
So it is OK to use a central DB cache for this.  Better would be a
central memcache of course but we don't have one of those (yet).
Merge branch 'master' of github.com:Stanford-Online/class2go into sef-grader-cache
grader cache: shorten cookbook readme (#2050)
Merge pull request #2282 from Stanford-Online/sef-grader-cache

Cache remote interactive autograder responses (#2050)
#2298 - Add "Username" as column title for Student Scores in-line report
Merge pull request #2299 from Stanford-Online/dca_2298_add_username

#2298 - Add "Username" as column title for Student Scores in-line report
Merge branch 'master' of http://github.com/Stanford-Online/class2go into dca_new_working2
made radio/check imputs line up next to labels in all cases
Merge pull request #2302 from Stanford-Online/giulio-input-styling

radio/check inputs now line up next to labels in all cases
add missing import to fix "-s" on regrade (fix #2303)
Merge branch 'master' of https://github.com/Stanford-Online/class2go into giulio-twitter-widget-2247