Improve state models for contents and content groups

Publish range

New properties have been added to set the range of published Contents of a ContentGroup instead of setting the state for each Content individually.

A Content is now published if its ContentGroup is published and for its index i inside contentIds applies:

firstPublishedIndex <= i <= lastPublishedIndex

or

firstPublishedIndex <= i && lastPublishedIndex == -1

Furthermore, the contentIds are now stored as a List (instead of a LinkedHashSet) because to entry index inside the Collection gained importance. The uniqueness is now checked by domain logic. This is an implementation detail and does not affect the API and persisted data. The use of a List makes checking if a Content's index is in the published range much easier.

Additional model and logic changes

Addtitional model changes:

• Added statisticsPublished and correctOptionsPublished to ContentGroup model
• Removed obsolete properties from Content model.
• Renamed Content's state properties for consistency.

Logic changes:

• Use answeringEndTime instead of a boolean value to check the permission for Answer creation.
• Logic which still used those properties has been also removed or refactored.

Migration handling

A migration has been added which moves some state information previously stored for individual Contents to ContentGroups. The new publishing state and range for ContentGroups is determined by first checking if all or none of the individual Contents are published (state.visible). In this case, the published is set accordingly and -1 is set for firstPublishedIndex and lastPublishedIndex. Otherwise, published is set to true and the range is determined by taking the first published Content and the last published Content in a row. Later Contents, which are not published consecutively, will be excluded from the range.

Furthermore, this migration adds additional properties which have been introduced to the models and removes obsolete ones.

Closes: #93 (closed)