Enable resources to have revisions

[edalzell]

Description

Would be amazing if resources (models) could have revisions, like entries do.

I realize this might require some PRs into Statamic, I'm exploring that.

Steps to reproduce

1. Try to update a model and have that be a revision

Environment

N/A

[edalzell]

Notes:

• copy most of how the eloquent driver does it
• Create a HasRevisions trait for this behaviour
• need a controller to handle the revision stuff, copy most from EntryRevisionsController
• figure out where the revisions panel goes

Statamic PR:

• Make Vue component out of the revisions div in components/entries/PublishForm
• (this might be a Runway thing) support multiple RevisionRepository classes so that Statamic stuff still works
• (maybe) make the revision-history components more generic so we don't have to duplicate them in Runway

[duncanmcclean]

Completed in #492.