Environmentalize public user registration

[p0ly]

There's a slur of parameters that can be configured in the variables.env, but there's no way to enable public registration without rebuilding the docker images for front and back. The variable is PUBLIC_REGISTER_ENABLED in the taiga-back conf and publicRegisterEnabled in the taiga-front conf.

This should be quick and easy. I'll gladly submit a PR if that's easier.

[w1ck3dg0ph3r]

Thanks! Yep, that's an easy fix, I'll consider adding more options to variables.env for convenience. You can just edit the configs and restart taiga-back container for now, no need for rebuilding images.