Improve exception handling

See merge request !52

Execption messages in API responses is disabled by default because they
can contain sensitive data. The config property
`api.expose-exception-messages` has been added and is described in the
developer documentation.

Error responses now expose exception details.

Exeption handling was broken by
56fe085e (GH-27).

Fix session import

See merge request !49

Readd an empty constructor for deserialization with Jackson. It was
accidentally removed a part of a code clean up in commit
685695a6.

Code cleanup

See merge request !48

Additionally, removed unused empty constructor

Additionally, made first use of it in CouchDBDao.

Additionally, remove unused loggers.

Bump Java source and target versions to 1.8

See merge request !47

SessionServiceTest.testShouldDeleteAllSessionData fails when run on
JDK 8.

This will allow us to use type annotations, lambdas, etc. Documentation
already states Java 8+ as a requirement.

Robin Drangmeister authored 8 years ago and Daniel Gerhardt committed 8 years ago

This fixes WebSocket communication for course sessions. (dg)

Refs GH-40.

Changed `security.facebook.enabled-roles` to
`security.facebook.allowed-roles`.

Clean up code

See merge request !46