Subscription expiry reminders are not always sent at the correct time

[jnugent]

Describe the bug
It's possible to configure subscription reminders to be sent at varying times in the OJS interface:

However, the scheduled task for the Subscription Expiry reminder is configured to only run on the first of the month:

	<task class="classes.tasks.SubscriptionExpiryReminder">
		<descr>Send automated reminders about subscription expiry.</descr>
		<frequency day="1"/>
	</task>

This leads to situations where journal managers are not sure if emails are going out. Suggestion here is to run this task once a day like review reminders if the intention is to allow shorter periods than one month before or after expiry.

What application are you using?
OJS 3.3.0.15