Queue new calendars for immediate import

[r-ferrier]

Fixes #2304

Description

After a bit of digging it seemed like the easiest solution here was to improve the notification shown in the header and to also immediately import a calendar's events on creation. That means that the user doesn't get to specify the date to import from on their first import (it will just default to now) but they can change that at any point after the import, and changing it to a date in the future will remove any events that have already been imported if they didn't want current events showing for any reason. We could add a field in the form to let them control when the import begins from on their first import, but that just felt like another thing for users to have to think about at the create stage when most of them probably don't really care about this, so I think this would be better as a new feature if it comes up again.

[ivan-kocienski-gfsc]

This looks almost okay but I think I'd be happier if this logic was in the model as an after_save hook. It can then be tested that saving a new calendar puts it into the queue

I am not sure if that the queue_for_import! method returns a truthy value if it succeeds (or a falsey value if it fails).

Also if the calendar doesn't successfully get queued (which ideally should never happen) then is showing a message in the flash the most helpful feedback?

[r-ferrier]

ok, have made those changes as requested - decided automatically queuing for import after any editing is always good - no one wants to be waiting around for their changes to be picked up after they've just made changes to something.

[r-ferrier]

ok I've made use of the after_create hook instead to prevent this queueing on edit - mainly because the number of tests failing when I tried to change this was so overwhelming and the way they're being run, would be hard to change, so I decided given the timeframe that just having this update on create is enough!

[ivan-kocienski-gfsc]

Looks good to me