Changelog: faster diffs and calmer notifications

Streaming diffs

Files now render as they arrive instead of waiting for the whole diff to resolve. Pull requests with several thousand changed lines become readable in about a second.

Symbol-aware search

Code search understands definitions and references for the languages we index, so searching a function name ranks its definition above the places that merely mention it.

One thread per pull request

Review comments, check results, and merge events for a single pull request now collapse into one notification thread that updates in place rather than adding a new row each time.