Free text validation is no longer done for abstentions.

This fixes a regression introduced in
9fc360d6.

Real-time updates for global statistics are unnecessary and put a lot of
pressure on the database. Data is now cached for five minutes. The count
of online users is still updated in real-time.

This simplifies analysis of monitoring data.

Configuration for Socket.IO has been refactored to support proxies and
replace ambiguous property names:
* Added API property `socketioPath` to `/configuration/`
* Added property `socketio.proxy-path`
* Removed property `security.ssl`
* Renamed properties:
  * `socketio.ip` -> `socketio.bind-address`
  * `security.keystore` -> `socketio.ssl.jks-file`
  * `security.storepass` -> `socketio.ssl.jks-password`

The default port to bind to has been changed from 10443 to 8090 since
SSL is not enabled by default. There is enough room (8081-8089) for
additional severs (e.g. `grunt run`) in development environments.

The security.*.enabled properties are now loaded as booleans.

The original implementation did not correcly implement the W3C
Recommendation (https://www.w3.org/TR/2014/REC-cors-20140116/). CORS is
now implemented based the CorsFilter provided by Spring MVC.

Origins allowed for CORS can now be set in the configuration.
Additionally, GET access to the following request URLs is allowed from
all origins:

* /
* /arsnova-config and /configuration/
* /statistics

The implementation (introduced by
2084f33a) was to permissive and posed a
security risk. As a consequence, cross origin requests requiring client
authentication are no longer possible.

Daniel Gerhardt authored 8 years ago and Tom Käsler committed 8 years ago

There is no need for a separate property. `questionValue` is used by
learning progress views.

Convert database user IDs to lowercase on account creation an login.
Additionally, LDAP UIDs are lowercased as a fallback when they cannot be
retrieved from the server.