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Cherry picked from master

The headers module might not be enabled by default.

Jetty 9.4.20 has a breaking change causing issues with Spring's
Websocket handling. This will be fixed in version 9.4.21.

See https://github.com/eclipse/jetty.project/issues/4020.

Closes #49.

Replace wiki link with relative repository link.

Update install guide for current versions for dependencies

See merge request !152

* Update CouchDB section for version 2
* Remove "or newer" for OpenJDK
  (ARSnova 2 does not run on JDK 11)
* Adjustments for Debian 9/10
* Install Tomcat 9

Update Travis CI config for 2.x

See merge request !148

* Remove notification settings
* `sudo` environment is no longer supported

An error was logged before destroying UserService class without any
reason.

The version set for Pac4j was previously ineffective because it was set
by spring-security-pac4j first.

The Spring Platform BOM has reached EOL, so we need to replace it for
future updates. This should not change any dependency versions.

https://spring.io/blog/2018/04/09/spring-io-platform-end-of-life-announcement

See GH-65.