[Feature] Add subscription enddate/renewal date

For subscriptions with a yearly contract that automatically extends for a year, it is very handy to get an notification in time, so you can consider whether you want to cancel or abort. Currently (v4.6.0) there is already a cancellation reminder, but that you set for a certain year, and thus each year have to update.

Proposal:

• allow configuring a subscription end date (like a start date)
• have an checkbox to indicate if the subscription auto-extends
• have a default setting for how soon in advance you want to be notified --> typically you need to cancel 1 month in advance. But sometimes you can already arrange a move to another company 3 months in advance, so you want to know earlier

Currently you have to:

• set a cancellation reminder and manually calculate the date in advance
• update the cancellation reminder for the next year (not major effort, but why not automate it?)