Improve redirects

[dandavies99]

Improves the general usability of the webapp by changing redirect behaviour.

• By default, users are redirected to the object they have just created after clicking "save".
• There is also a "save and add another" button which leaves the user with a blank form and the usual confirmation success message.
• "Add new" buttons are at the bottom of results tables so users don't have to navigate away or use the navbar to do this.
• The homepage contains a simple table of data types and their descriptions with links to table views and new object views.

To be merged after #99