Comment 33 for bug 1855780

Thank you to Mike, Galen, Rogan, Chris, Terran, Christine, Ruth, Jennifer, Jessica, and Erica. I have pushed this to master for inclusion in 3.9.

I took the opportunity to make two small follow-up commits: one just removes some unnecessary whitespace, the other makes the dropdowns behave as Christine expected in comment #22 point #4 and Erica expected in comment #32 point #6.

To address some of Erica and Christine's points:
* Comment #32 point #7 (owning library currently doesn't show as a required field) is a manifestation of an existing bug: bug 1933498.
* Comment #22 point #5 and #32 point #7a (An "update succeeded" toast appears when the update didn't succeed) is also present in a few other screens: bug 1913816 and bug 1912553, for example.
* Comment #32 point #2: those fields were new to me, too. They were introduced very recently, in bug 1207533. I don't know enough about that development to understand how staff will be interacting with them. Perhaps somebody with more knowledge of them could open up a follow-up bug -- I could see some kind of tooltip being helpful for these fields.

Also a note that I opened up bug 1951307 as a small UI follow-up.

Apologies to Jessica: I neglected to include your signoff on the relevant commits!