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
Split up rake db:seed and rake db:seeds:fetch

rake db:seeds:fetch can be invoked by hand now, and rake db:seed no
longer automatically calls it. This allows local edits to seeds.sql

When seeding database, do not drop and recreate the local database
according to an online schema - instead keep the existing local
database. This prevents changes from local migrations from being
overwritten when seeding the database
Fixed language path for languages with non-URI chars
Merge pull request #2071 from bluerogue251/allow_local_changes_to_db_for_seeds

Allow local changes to db seeds
Merge pull request #2073 from ErikSchierboom/languageuri

Fixed language path for languages with non-URI chars
Fix markup of comments

Fixes #2072
Make code sticky on scrolling

only for desktop
Merge pull request #2074 from jalyna/fix-mentions

Fix markup of comments
Update state of all submissions when done

When a user had one or more submissions whose state was not
`superseded` and that user decided to post a new submission and close
that exercise. Then the latest submissions state would correctly get
changed to `done`, whereas all the old submisssions would not have their
states updated to `superseded`. This has now been fixed and updated to
correctly change the state of old submissions.

* Add test that state of previous submission changes
* Update completion to change state of old submissions
* Update completion_test to assert that old submissions states are updated
* Remove unused variable in completion_test
Merge pull request #2017 from Adam89/update-submission-properly-when-input-needed

WIP: Add test to assert state of submission changes
Merge pull request #2075 from jalyna/sticky-code

Make code sticky on scrolling
Differentiate looks good from nitpicks