Admin style

[tomchristie]

Refs #1904.

Very much a work-in-progress right now, but may as well make it visible as we go.

Going with a simple template-only based approach at the moment, that should render fine with existing APIs.

Considering also having a more fully featured third-party admin pacakge that does more opinionated things such as built-in Core API hypermedia support, and proper browser behavior on eg failed validation, which this version will need to handle in a more basic way. (Last part of that actually not as much of an issue as I'd thought)

TODO:

• Errors need to correctly respond in modals.
• Links in list view (prob as a control, rather than hyperlinking a field), also have a clean list fields API.
• Pagination control currently showing even for a single page.
• Hyperlinks. (Render as URL in JSON, but as text or other value in admin)
• [ ] Resolve modal flash. Passing on this for now.

For 3.3.0:

• Clean up detail vs list pages and templates (extend base)
• Documentation on overriding.
• Don't always just include the format in outgoing URLs - only do so on defined links in browsable API.
• Actions and naming of buttons etc (Inc ensuring internationalized), also clean modal overriding.
• Messages on edit operations.
• Strip 'format' for the main HTML representation.
• Form modals to only include the name part of hyperlinks in select elements.
• SlugRelatedField should hyperlink.
• Update heroku example
• Filters in HTML.
• Document the HTML form API. (Possibly with eg nesting and/or list support)

[tomchristie]

Going to pull this in now.
Think it's about time we wrap this up as a 3.2 release, with refinements to follow in 3.3.