Add source branch and hide common target branches in PR dashboard

[kroeschl]

Justification

When looking at the PR dashboard, I frequently want to see the name of the source branch for a given PR. I also almost never care about the target branch, because it's usually just main or master.

Implementation

Add the source branch to all PRs. Hide the target branch (and related text and icon) if it's main or master. Also abbreviate user branch names like users/kroeschl/foo to u/k/foo to save space in the dashboard. Actually update the README and update notification.

Testing

Viewed some PRs into main and a test PR into not-main to verify rendering:

Also verified that notification is updated and actually displays in browser:

[alejandro5042]

Hey! Thanks for this draft PR! Overall, I like it. Also, thanks for updating the popup.

I'm not sure about the rewriting the branch name--it would then display incorrect/misleading information. I'm down for shortening, but I'd want it to look different.

How about this?

👤/my-branch

[kroeschl]

How about this?

👤/my-branch

I like that idea. Something like this? Should we do something different for not-me? Or just not shorten branches for other users?

Or we could use … to make it more obvious without needing to handle self vs other users differently.