WIP: Kuhsaft v3.0.0

This PR serves to coordinate efforts towards the next major Kuhsaft release.

Major Topics for the 3.0 release are:

**Better out of the box experience**

- [ ] Include default Frontend Partials/CSS/JS based on foundation, allow easy theming and overrides
- [ ] Move from Bootstrap to Foundation for Backend UI, revamp UI with focus on usability
- [ ] improve/clean up documentation
- [ ] improve installer and update tasks

**Architecture changes**
- [ ] Handle Custom Models. This will either be done via merging in stuff from shoestrap  (#230) or by integrating nicely with active_admin.
- [ ] Multisite capability (#213)
- [ ] Mountability on non-root paths (#130)
- [ ] Revamp Placeholder/PageType/Bricks logic (#174)

**Misc new features**
- [ ] Media Pool (#181)
- [ ] Basic workflows and user roles (#180)
- [ ] Preview Page (#231)

**Meta feautres**
(not really CMS features, but planned alongside 3.0 release)
- [ ] Demo Page (#187)
- [ ] renaming/branding
- [ ] PR Page