Always set `creationTime` on session creation. Sessions with invalid
timestamps might otherwise be deleted by the scheduled cleanup.

Fix NPEs

See merge request !73

Fixes a regression introduced with commit
dd749370.

Refs arsnova#85
Refs !16

Fixes a regression introduced with commit
d74ef999.

Refs arsnova#85
Refs !25

Fixes a regression introduced with commit
7207bad9.

Fixes arsnova#84

This fixes compatibility with CouchDB 2.

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

Our version of CouchDB4J has been updated to fix and improve type
handling for database keys. Strings are no longer converted implicitly
to integers or booleans based on their content. This lead to bugs if key
and property values were compared.

Use relative links to repository files in documentation

See merge request !64

Also block local access using IPv6

See merge request !63

Rename CI repository variable for GitLab 9.0+

See merge request !62

Migrate to Pac4j framework for OAuth handling

See merge request !59

Since we do not use a SecurityFilter, the previously visited page is not
saved in the session.

This improves/fixes compatibility with third-party OAuth APIs.

Improve handling of context path prefix

See merge request !61

Consistently prioritize the `api.path` setting before falling back to
`servletContext.getContextPath()`.