User cache not flushed on model save

[lcharette]

This means all changes to the CurrentIssue are not displayed until cache expires or is manually flushed.

To replicate issue:

Go to /account/settings and change the current user first name. The change won't be displayed until cache is cleared (default to 2 mins).

Can fix by setting the cache.user.delay to 0 temporarily.

This should be done automatically using the following event :
https://github.com/userfrosting/UserFrosting/blob/master/app/sprinkles/account/src/Database/Models/Events/DeleteUserCacheEvent.php#L30

Can't be replicated in automated testing as caching is disabled by default...

N.B.: Old cache was 120 minutes, but Laravel cache remember method changed between 5.4 and 5.8 from minutes to seconds. So now it's 120 seconds (2 mins). This two need to be addressed