CWS: read available localizations only at startup.

This avoids many file system read operations, that could (in theory)
be a DOS attack vector.
Add communication test for Java.
Deny statistics refresh if previous request was not finished yet.
Adjust comments on which languages are supported.
Fix TwoSteps task type.

Since it was never tested, we let several non-compatible changes make
it non-working. This should at least make it work (tested
manually). Writing some tests for it will require touching the
infrastracture of cmstestsuite to allow more than one file per
submission.
Don't limit width of admin panel.
Typo.
Amend AWS tables to take the space they need.
Rename CSS styles to saner names.
Fix ContestImporter.py to work with .zip files.
Print pretty warning when exporting non-existing contest.