Free text validation is no longer done for abstentions.

This fixes a regression introduced in
9fc360d6.

Clear caches at fixed time intervals



See merge request !30

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.

import / export freetext questions



See merge request !29

This simplifies analysis of monitoring data.

Add proxy support for WebSocket connections

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.

See merge request !27

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.

Use preexisting questionValue instead of freeTextScore

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

See merge request !26

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