Add initial 'try it now!' functionality to the document browser.

[blanquer]

This is a longer epic, but we need to add some basic support for issuing API requests (and visualizing responses) from the document browser.

There are a lot of design considerations to be had, which should be fleshed out in the mailing list:

• Form design overall
• Integration with custom types
• Authentication support
• look and feel
  ...

[careo]

We'll also need the CORS pre-flight request support.

[gampleman]

There is some initial work done by @aliscott here: https://github.com/aliscott/praxis/tree/api_docs_try_it

As far as I understand, the main work remaining there is working out good support for type editors (built in and custom). Currently you will need to type in raw JSON for the attributes and payload.