Run Linguist and diff highlighting as a seperate service
## Why
My motivations for this are mixed:
1. I noticed that Linguist adds significant time rails startup, which slows down development as well as spawning new production workers
2. I think someone mentioned that diff highlighting is part of the reason why some pages are slow. Sometimes I get fed up waiting for an MR diff to load and end up appending `.diff` or `.patch` to the URL and getting a very quick response with everything I wanted.
## Proposal
1. Whenever we render a diff from rails we'd first render it as plain text and then make a second API call to get the highlighted diff
2. This API endpoint would optionally be on a separate worker, allowing other workers and the dev environment not to load Linguist at startup
issue