Filter sessions on the DB side to check if key is available

See merge request !58

Daniel Gerhardt authored 7 years ago and Daniel Gerhardt committed 7 years ago

This should massively reduce the time needed for session creation.
Additionally, the `Transactional` annotation has been removed because
it did not have any effect in this case.

Log level for DB logging was overriden by the fallbacki value in some
cases.

ARSnova no longer supports JDK 7 which is still the default for
Travis CI Java builds. `sudo` is disabled to speed up the build start up
time.

Throw and handle NoHandlerFoundException for unmapped paths

See merge request !57

Remove obsolete config

See merge request !56

There should not be any Servlet container around which does not support
JSON by default. The backend does not have an AppCache manifest so a
mapping is not needed here either.

Since error handlers use JSON for responses there is no need for HTML
error pages anymore.

Add multi-version API support via media types

See merge request !55

API v3:
* Media type: application/vnd.de.thm.arsnova.v3+json
* Serializes date as strings conforming to ISO 8601.

API v2:
* Media type: application/vnd.de.thm.arsnova.v2+json
* Serializes dates as timestamps including nanoseconds.

An indentation setting has been added: `api.indent-response-body:false`.

Exception logging

See merge request !54

Add messages to exceptions and slightly refactor exception handling.

Exceptions are now always passed to the logger. The logger decides based
on configuration whether and how they are logged. Logging messages were
adjusted for consistency.

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.