New Type 'Class' in userinterface

[EeveesEyes]

Reason (Why?) The new type 'class' should be represented like all other classes.

Solution (What?) The following changes need to be made:

• [x] new class interface in data model
• [x] new class page, containing subclass_of relation
• [x] new facet for type on home page
• [x] new entities-page /classes
• [x] add translations for other languages Edit:
• [x] add representation of 'class' to the artwork page
• [x] rename all string-representations in the translation files from 'class' to 'type'
• [x] add class to search suggestions

Relation to other Issues Depends on #452 Mentioned in #451

Acceptance criteria Classes can be browsed

[EeveesEyes]

I forgot to add a step for the representation of 'class' on the artwork page. An array with classes should be added, as well as classes to common tags at the bottom of the page. One addition: Please rename all string-representations in the translation files from 'class' to 'type' (and accordingly for de,es,fr & it). In code it should stay 'class'.

[EeveesEyes]

On a side note: subclass_of is stored in an array, which should be considered in the frontend :)