Make red highlight of `ProxyError` messages configurable via env var

[teremterem]

The common/utils.py::ProxyError class has a default value for highlight parameter in the constructor as True.

1. Read this default highlight value from an env var (optional, with True/true/yes as default) into a global variable in common/config.py.
2. Use this variable to set highlight value for ProxyError instances upon their construction. Explicitly passed values should override the value from env var, however.
3. Update .env.template with this env var commented out and an explanation in a comment near it, similar to how other optional env vars are mentioned in this file.

[teremterem]

A temporary solution to this problem was provisioned by PR #107. To fully close this issue, however, the comments from #107 should be addressed in the form of a new PR.