group queued releases in build-queue page

[syphar]

Coming from this zulip discussion.

Especially when we do rebuilds of many crates (and even more when we do #464 ) the build queue page can get quite long and probably confusing for crate authors.

One idea would be to:

• group the queue into sections by priority groups (normal prio, deprioritized, perhaps manual rebuilds, automated rebuilds)
• hide the "automated rebuilds" by default, perhaps only showing the count, with a link to show them.