Pull all variants onto the default branch

[thunderbiscuit]

I am proposing this change because while the "one branch per variant" idea is pretty clean, it has two downsides:

1. Discoverability of the variants. Devs need to read the readme to realize we maintain multiple versions of the app.
2. Contributions not on the default branch are not counted by GitHub. This lowers the incentive for devs to work on other versions of the app, or to propose entirely new ones. All commits made to a branch that's not the default will simply not appear in their contribution graph. This usually matters for (i) students and devs building their resume, (ii) people working in open source where they need to showcase their work to apply for grants and renewals.