Create new MetaData and Content models to modularize Site

[esalling23]

Includes work outlined here for Meta data implementation:

• Site title/description meta data squeezer-client#11

[esalling23]

MetaData and Content will both potentially require FormData to send images to the server. In wanting to keep each object's changes separate rather than merged into 1 object, we'll need a different FormData object for each, and FormData does not support nesting.

Keeping FormDatas separate is likely also better from a scalability approach to support more & different file uploads & being able to better determine where those files should go.

Q: How can we send each FormData obj to the server as a separate object?

We'll change the updateSite request into just handling site generation. For updating the other parts of the site, we will have separate requests for Content, MetaData, and Theme models.

On the client side we can chain requests to first update Theme changes, then MetaData, etc. Order shouldn't make much of a difference.

In future work we could consider local changes & only update site objects as necessary, ie: only update Theme if that's the only values changed.