Refactor elements

[jochenklar]

• Add page model
• Use m2m relations between elements
• Add a new react front end at /management/

[jochenklar]

This needs to be done on postgres to reset the autoincrement counters:

SELECT setval('questions_page_id_seq', (SELECT MAX(id) from "questions_page"));
SELECT setval('questions_questionset_id_seq', (SELECT MAX(id) from "questions_questionset"));

we need to add that to the update notes.

[coveralls]

coverage: 89.854% (-0.9%) from 90.792% when pulling 228c05d on refactor_elements into 2d83968 on dev-2.0.0.