This PR brings back https://github.com/sourcegraph/sgtail back in `sg`,
plus a few adjustments to make it easier to use. I'll archive that repo
once this PR lands.
@camdencheek mentioned you here as you've been the most recent beta
tester, it's more an FYI than a request for a review, though it's
welcome if you want to spend a bit of time reading this.
Closes DINF-155
## Test plan
Locally tested + new unit test + CI
## Changelog
- Adds a new `sg tail` command that provides a better UI to tail and
filter log messages from `sg start --tail`.
