Make some env vars optional for easier dev setup

[carols10cents]

Right now cargo_registry::env panics if there isn't a value set for an env var, so you can't run the server locally unless you set values for everything in .env.

If you're just working on part of the backend that you don't need users logged in for, there's no reason to have to set up github tokens and all that.

I tried to make this easier to set up by setting the values in .env.sample to "", which gets around the panic, but apparently that caused problems with zsh.

I think it'd be better to have another function, like env_with_default or something, and maybe warn that functionality that needs that env var isn't going to work?

I'm happy to do this, just not right this second, so I wanted to open this to remind me (and maybe entice someone else to do it, i don't think it would be very hard #famouslastwords)

[alexcrichton]

Sounds good to me!

[kelcecil]

I'd be happy to take this task.

[kelcecil]

My thought is that we may want to still panic for certain environment variables not being set where we absolutely can't progress. TEST_DATABASE_URL, S3_* variables, and GH_* variables would all be okay for defaults, but the Postgres URL and GIT_* variables would pretty much be required and should panic if not set. Thoughts?

[alexcrichton]

@kelcecil yeah sounds good to me. We may want to assert that in production everything is set, but otherwise sounds good

[carols10cents]

I think we fixed this. If anyone hits this problem, please reopen!