Notifications: use toasts for temporary notifications

[agjohnson]

We discussed a number of plans for notifications at our offsite, mostly stemming from some user support issues and some related problems we've had with build and project UX. The first change that I'd like to make is to replace Django default level messages with "toast" messages. These messages are floating in a corner of the page, and only display for a few seconds before disappearing. All of this is configurable in addition.

This is useful for a few messages that get shown to the user, but right now take up a good amount of screen real estate between the header and the page content. In addition, these don't disappear until the user navigates to a new page. Messages in this class are "Log in successful" or other versions of form failure/success.

This would leave us with sticky/persistent Django messages next, and we did discuss some options for moving/changing these messages and their UX. It would not need to be part of the first pass though.

[agjohnson]

I started a proof of concept on the train, I'll try to push my work up here this week. I did try to separate out default messages into toasts and sticky/persistent messages into items under the bell icon in the head menu, but this did prove hard. I'll punt on this change, we need to discuss the best way to handle this.

The main place I stumbled with splitting out sticky messages is that the bell notification menu requires two filters on the messages context variable, for unread messages and sticky/persistent messages. This probably should happen in a context manager or perhaps as a template filter.

[agjohnson]

Discarding the plan to repurpose sticky/persistent notifications for now, the change to toast notifications is actually quite small.