Partial re-style of asset form

This is a large PR which changes the styling of the asset form somewhat. It is worth reading each commit's message in its entirety, especially dbbc3682 which implements the bulk of functionality.

The form has not yet been fully implemented but this PR is being opened early to allow merging of the re-worked plumbing.

Aside from dbbc3682 the remainder of the commits are small changes to functionality required to implement the AssetForm refactor.

Partially addresses #16 (closed)