Move support for Beaver Builder into core

**Is your feature request related to a problem? Please describe.**
Users may not trust installing an experimental LifterLabs plugin to get Beaver Builder support.

**Describe the solution you'd like**
- [ ] Move the support for Beaver Builder into core.
- [ ] In core, check if the Lab is installed and enabled and if so, do not also load the core lab
- [ ] By default, remove the default Course content at the bottom of a Course post (similar to Elementor)
- [ ] If possible/quick, add in the default BB modules to a new Course post when you start editing it in Beaver Builder.
- [ ] Ensure if labs + core with BB moved over is installed, BB still works as expected.

**Describe alternatives you've considered**
N/A