Entity type management

Refs https://redmine.teklia.com/issues/2687
Depends #1205 (closed)

We now need a full CRUD interface to manage the entity types, exactly like we manage corpus types. The component should be displayed as a new tab in the project information, titled "Entity types".

A table with the following columns will list all the existing entity types on a corpus:

• ID
• name
• color (displayed as HTML code and as a small sample of said color)

A corpus administrator should be able to:

• create a new type by specifying only the name
• create a new type by name and color (through the standard HTML color picker)
• delete a type (provided it's unused - do not make assumption, simply display correctly any error raised by the backend)
• edit an existing type