Add /stats endpoint for rooms with content group stats

See merge request !158

The endpoint currently returns the number of contents of the room and a
list of all content groups with their number of contents. Additional
stats like total answer and comment are not set yet and have to be added
later.

Refs #45.

Conditional beans

Closes #35 and #50

See merge request !157

Beans related to authentication providers are now only initialized if
they have been enabled via configuration.

Besides eliminating memory usage and initialization time of the beans
this fixes issues caused by filters of disabled authentication
providers.

Fixes #35, #50.

Make deletion of user profiles cascading

See merge request !156

The deletion of users now triggers a cascading deletion for rooms owned
by the user.

Extract code coverage from Jacoco for GitLab

See merge request !155

Remove redundant modifiers and add Checkstyle rule

See merge request !154

Remove unneccessary imports and add Checkstyle rules

See merge request !153

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

Remove obsolete PropertySourcesPlaceholderConfigurer bean

See merge request !151

An PropertySourcesPlaceholderConfigurer bean was still set up which
tried to load the legacy properties files.

Refs !137.

Fix persistance related bugs for contents and answers

See merge request !150

* Fix delayed answer creation
* Fix deletion of all contents of a group
* Fix deletion of all answers of a content
* Fix answer count for groups
* Initialize new answers with round 1

Refactor handling of content groups

See merge request !141

Use updated content group handling and remove all references to the
old `variant` property of contents/answers.

The following /v2 endpoints are now fully implemented:
* Deleting all contents of a group (/)
* Retrieving ids of unanswered content in a group (/unanswered)
* Deleting all answers of contents in a group (/answers)
* Counting answers of a group (/answercount)

The `ContentFindQueryService` now supports filtering contents of a room
that are not part of any content group.

Content groups are now a separate entity with their own repository and
service and endpoint. A content group can no longer be set when creating
a new content. Instead the new API endpoint has to be used to add the
content to a group.

Update Travis CI config

See merge request !149

* Build and test on OpenJDK 8 and 11
* Upgrade to xenial image (bionic causes issues with JDK 8)

Update Travis CI config for 2.x

See merge request !148

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

Refactor account activation and password reset

Closes #47

See merge request !146

Account activation is now using the ban logic as used for login failure
logic.