port fault tolerant syntect_server implementation to sourcegraph/server

Created by: slimsag

After https://github.com/sourcegraph/sourcegraph/issues/5406 syntect_server was made much more stable in cases of it going rogue. We can also improve this failure case in sourcegraph/server by using http-server-stabilizer in the same way, but with one worker.

This will require doing some refactoring to share some common Dockerfile logic with cmd/server/shared.go so we do not have an ultra-long list of tediously-copied env vars. See https://github.com/sourcegraph/sourcegraph/issues/5406