Fix startup w/o migrate-from property

Closes #56

See merge request arsnova/arsnova-backend!182

Startup failed when `system.couchdb.migrate-from` was not set. This was
a regression introduced by !174.

Fixes #56.

Gridimage content format

See merge request arsnova/arsnova-backend!180

Answers for `GridImageContents` are stored as `ChoiceAnswer`s.

The model of grid image contents has been overhauled to store data more
efficient. Fields are now represented as a single index int value
instead of a semicolon separated String. Instead of storing all
selectable fields now only the correct ones are stored.

Further changes:
* Field size instead of scale factor and zoom level for grid
* Normalized values for offsets and field size
* A single scale factor value for the image
* Rotation in degrees
* Removed coloring settings

Fix NullPointerException caused by AnswerStatistics

See merge request arsnova/arsnova-backend!181

A NullPointerException was caused by RoundStatistics with uninitilized
lists.

Continue aborted migrations with help of saved state

See merge request arsnova/arsnova-backend!179

The error message includes the list index so it is easier to find the
document causing the problem.

Migrations can now be split into smaller steps. This change and
additional stored migration state data allow aborted migrations to be
continued.

Add update support to content group endpoint

See merge request arsnova/arsnova-backend!177

Content groups can be updated by sending the updated entity via PUT
request to the /room/{roomId}/contentgroup/{groupName} endpoint.

Fix HTTP connection leak by database index creation

See merge request arsnova/arsnova-backend!178

Fix NoSuchElementException in group stats endpoint

See merge request arsnova/arsnova-backend!176

A NoSuchElementException was thrown when the stats endpoint was
requested for a room w/o any content groups.

Fix NPE in migration of text content format

See merge request arsnova/arsnova-backend!175

Fixes a regression introduced by arsnova/arsnova-backend!174.

Improve migrations

See merge request arsnova/arsnova-backend!174

The solution text is no longer preferred over the hint text if it is an
empty String.

Migrations are once again enabled if `migrate-from` property is set.
Code has been refactored to use `@ConditionalOnProperty` instead of a
custom implementation for `@Conditional`.

* Add handling for OIDC, SAML and anonymized profiles
* Remove prefixes from loginIds

Content groups are created based on question variants of the v2 question
documents.

Use Object instead of wildcard for extensions' inner Map

See merge request arsnova/arsnova-backend!173

Wildcard types should not be used as return type.

Add additional text properties for content

See merge request arsnova/arsnova-backend!172

An additional text and title can be added to contents which can be used
for multiple purposes based on the content format. These properties are
also used to migrate solution and hint properties from v2.

Add SAML authentication support

See merge request arsnova/arsnova-backend!171

Those classes are now used for OAuth, OIDC and SAML authentication
handling, so the Oauth prefix would be confusing.

Fix CAS callback path

See merge request arsnova/arsnova-backend!170

While it didn't cause any issues it doesn't make any sense to have the
CAS callback path under `/oauth`.

LDAP and SSO auth endpoints

See merge request arsnova/arsnova-backend!169

HTML/JavaScript code is output by the SuccessHandler to close the
browser window if SSO login was opened in a popup.

A short lived cookie is set for the temporary JWT. The cookie will be
deleted once the token is manually refreshed.

Because the API's callback isn't directly called by the frontend, it
doesn't have access to the response. I found three options to pass
authentication to the frontend: via URL (query param/fragment
identifier), JavaScript in the callback response or a short lived
cookie. The cookie is easy to implement and doesn't require any
knowledge about the frontend.

A `refresh` parameter has been added to /auth/login to create a new JWT
with an extended `expiredAt`.