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.

* Update MathJax URL to use version 2.7.5
* Piwik was renamed to Matomo

Add support for OpenID Connect authentication

See merge request arsnova/arsnova-backend!118

Only OIDC providers that support configuration discovery are supported.

Google OIDC

See merge request !117

Google shuts down its Google+ Sign-In.

Add API endpoint to create copies of public pool sessions

See merge request !116

Fix import format

See merge request !115

Daniel Gerhardt authored 6 years ago and Daniel Gerhardt committed 6 years ago

The field of the inner class is kept for compatibilty reasons.

Delete inactive users

See merge request !102