Group notifications into one when more than one car is posted for a source

[sphars]

If a source has more than one new car, change the notification to be more generic ("new cars posted at "), and link it to the source's homepage. Alternatively, list them in the notification, but may be unwieldy with > 5 cars or so. Should also help to not send multiple requests to ntfy.sh at once.

Once more sources are added, this behavior may be further adjusted.

Reasoning: AFCU likes to post new cars weekly (usually Wednesdays) so when the script runs, users receive multiple notifications at once.