Show Feedback guide on demand

Fixes #2060
Create a data-heavy footer

Fixes #1840
Merge pull request #2061 from jalyna/fix-feedback-guide

Show Feedback guide on demand
Merge pull request #2062 from jalyna/create-data-heavy-footer

Create a data-heavy footer
Add API endpoint for specific problem/solution by uuid
Retina #brand logo in the navbar

I replaced the `e_small.png` and `e_red_small.png` images with the same images
but with their sizes doubled. This way the logo looks nice on retina screens. I
also had to modify some CSS in order to constrain the images (which are just
`background`s) into a fixed size, since the previous CSS relied only on the
actual size of the images.
Merge pull request #2068 from whatyouhide/retina-stuff

Retina #brand logo in the navbar
Add a retina favicon

I added a retina favicon.ico (32x32). I also added a 180x180 Apple touch icon
(as per iOS 8 specs).
Merge pull request #2069 from whatyouhide/retina-favicon

Add a retina favicon
Avoid 500 error when guessing exercise keys
Add migration for new solution field on submissions

This is the first step towards being able to submit multiple files.
See #1165.
Clean up rake tasks
Migrate existing submissions into 'solution' field

This can be run at any time, as running it again will only affect new
submissions. That said, it probably isn't worth running it until the
controller that accepts submissions stores the solution both in the 'code' and
'filename' fields, as well as the 'solution' field.

See #1165
Fix key names in submission api endpoint