This used to be broken to a point where it would crash any page with a
`<<youtube:…>>` magic link. The gods know whether this template is how
it should be, but we get it from the YouTube sharing/embedding panel
so it should be halfway OK.

This is for consistency (sorta-kinda) with Markdown image
inclusion. (Markdown allows different texts for `alt` and `title` but
we're still looking for a use case.)

Having line breaks in a caption now works as intended, and we're
fixing an issue with the thumbnail box width having `pxpx` at the end.

This led to problems with some images.

This makes a lot more sense.

Until we figure out how to put the dropdown box of the autocompletion
widget on top of the rest of the modal, this should help us make more
of the autocompletion stuff visible.

The problem with the PostgreSQL LISTEN/NOTIFY approach is that it ties
up database connections that don't seem to be freed even if the
browser moves to a different page. There are also various issues with
running the whole ACE4 app in uvicorn (e.g., Sentry seems to have
stopped working, which sucks.)

Instead, we're using a purpose-built async web server to handle
SSEs. The `https://my.strathspey.org/dd/list/:id/events/` URL is
redirected (using Caddy) to that server, and the long-lived
asynchronous HTTP connections that SSEs relies upon are handled by it
instead of Django. See the `ace4-notify` project for details of this
server.

This is a weird error but should not produce an exception.

Fixes STRATHSPEY-6

When switching between different templates in the new-dance-list
modal, selecting a template with no date or date template would cause
an `UnboundLocalError`.

Fixes STRATHSPEY-8.

This would otherwise lead to problems when calculating the times for
an item, because the last item in the dance list would cause an
out-of-bounds access in the list of item durations returned by the
database.

This also replaces most of the JavaScript code in `list_edit.html`
with HyperScript.

This automatically re-reads a dance list if its editing page is open
when a dance is added to it from a dance detail page.

This now uses a toast to notify the user that the dance was added.

These appear in the bottom right corner of the window and go away
after a configurable delay.

Also, a collection settings form.

The datatables weren't being properly initialised. [Viktor Lehmann]

This didn't even have a proper URL connected to the pulldown menu
button. [Viktor Lehmann]

This was inadvertently omitted when recordings were added. [Cord Walter]

Times are really calculated in the browser but we want to avoid a
distracting “flash” effect when an item is reread with times set to
“00:00” and the real times are only inserted later, so we deliver the
correct times from the backend already.