[5.5] Add an everyFifteenMinutes function to the ManagesFrequencies trait t…

[mmccaff]

I often want to schedule things to run four times an hour (every 15 minutes).

I know that using ->cron('*/15 * * * * *') is an option, but would rather use an everyFifteenMinutes function, like the existing everyMinute, everyFiveMinutes, everyTenMinutes, everyThirtyMinutes, etc.

I saw a few people ask for this, so thought it'd be a nice simple PR to add to the framework.

[paulofreitas]

I'm not against this but I think we're going in the wrong way... What was really missing here was a new every() method like this implementation. This would have covered all those specific cases. 👍

[ezaca]

Way too specific! Shouldn't use PHP relative formats through the class DateInterval for that?

E.g.: ->everyInterval('15 minutes')