Feature/add intro boxes

[acholyn]

Summary: Have added introduction boxes to works and books. This involved changes to the templates, forms, models and views (and migration). Related to #376

Models

• have added introduction - 1:1 relationship with TextObjectField instance
• have added plain introduction - plain text version for search purposes
• this should be the same across ant/work/book
• when a new work/book is created, it should create an introduction (TOF) object associated with it

Views

• new update introduction views based on update views- the work one follows the same pattern as the ant one in which it just uses the detail page with functions for handling an update. The book one is more complicated as books have no detail page so it redirects to the creation form but will act as an update. There is a filler placeholder (this is empty) that will create a text object associated with the work/book if it does not already exist

Forms

• Have taken the existing antiquarian introduction form and created a base form which the ant/work/book introduction forms are based on
• have added introduction fields to book and work (create) forms

Templates

• have added the boxes - which just makes use of the preexisting text_object_preview.html partial, same situation for the rich text editors for adding/updating the introduction content
• have added add/edit buttons depending on if the introduction exists (has not been created yet) and if it is less than a certain length (the content at base has some empty tags - this might be a good idea to update)
• similarly to above, the boxes showing the introductions should not show if they are empty
• with the new editing states, they also have tab descriptions

Tests

• check that TextObjectField instances associated with the work/book introduction are created/deleted with the work/book (models)
• check that the content is properly updated and that it is in the editing mode (views)