Redesigned notifications

Notifications have been redesigned to be easier to scan and act on.

• Who did what. Each notification shows who acted, with their avatar, and a snippet of the comment when there is one.
• Infinite scroll. The full notifications page keeps loading as you scroll, so older notifications are always within reach.
• An unread-first tray. The tray in the header shows what you haven't read yet. Click one and it's cleared; Mark all read empties it instantly.

Also in this release:

• Back where you were. If you're signed out while opening a link to a ticket or board, you'll land on that page after signing back in, instead of the projects list.
• Better doc comments. Threads and replies show newest first, clicking a comment scrolls to the text it's about, the open thread's highlight stands out clearly, and the text you're commenting on stays highlighted while you write.
• Links in descriptions work. Clicking a link in a ticket's description or acceptance criteria now opens the link instead of switching into edit mode.