Specify HTTP proxy for install

[mtlynch]

For customers that keep TinyPilot on a restricted network, they can't update through the web UI.

It would be helpful in these situations to allow the user to specify an HTTP proxy on the update dialog so that TinyPilot does all network requests through the HTTP proxy instead of trying to access the Internet directly.

We should be able to set the HTTP_PROXY environment variable, but the tricky part is that we call sudo at least twice (a, b) in the update path, which would drop environment variables. We could preserve the variable with sudo --preserve-env=HTTP_PROXY, but maybe there's another solution.