Made links relative for GitHub compatibility.

The Netty version set by Spring IO platform's BOM is not compatible with
netty-socketio.

Use relative links to repository files in documentation

See merge request !64

Rename CI repository variable for GitLab 9.0+

See merge request !62

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.

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