Allow deleting subscriptions

[chvp]

This is currently possible by crafting a special request, but that's quite annoying. Some button (e.g. on the edit screen?) would be handy.

[robbevp]

@chvp Did you delete one with such a request? I saw an error in sentry where the RssFeed still existed but the Subscription was deleted and our job to update the rss feed failed

Some loose thought:

• I've been thinking about adding a pause to feeds as well - so that the entries aren't all deleted, but you can choose to not get new entries for a while
• What should we do with newsletter subscriptions? Should we just return an error to the sender? Or should the user have some way of configuring where these emails should go?

[chvp]

I did yes. (Two in fact, but one of those had been in the error state for a good while.)