Improve comment download

[schultyy]

Downloading comments from big repos takes quite some time. As a user I don't see any of the progress until the web worker finishes and shows the banner "all comments downloaded".
Also right now it first downloads all comments and then puts them all into the database. Instead it would be nice to just put a bunch of comments into the database right after they've been downloaded. So the user gets to see at least some comments already while others are still being fetched.