Add 'stories' feature to show multiple experiments on the same set of models

[MichaelClerx]

This is described in more detail in https://docs.google.com/document/d/1WikWmKCTyktKrtj9WyhL_y5iWlcE9IQMDR6sJmHmXpg/edit?ts=5ef224a1#

[MichaelClerx]

Captions for figures

[jonc125]

This is described in more detail in https://docs.google.com/document/d/1WikWmKCTyktKrtj9WyhL_y5iWlcE9IQMDR6sJmHmXpg/edit?ts=5ef224a1#

[MauriceHendrix]

I'm adding functionality to make groups of models. Attached you see:

1. overview of model groups
2. creation form (get there by clicking create in overview)
3. edit form (get there by clicking on model in overview)
4. overview for a different author without create privileges but with some shared model groups
5. edit for for a shamed model for this author: visibility is disabled and models they don't have viewing rights for are disabled.
   The reason I decided to include model groups that you collaborate on on the overview is that I wanted a way for collaborators to edit them.
   Let me know your thoughts @MichaelClerx @jonc125 @mirams @agrawal-aditi