Double-check that superuser accounts can delete sources and annotations through frontend

[jgonggrijp]

It is hypothetically possible for trolls to self-register an account and vandalise the interface by creating bogus, potentially offensive annotations. Currently, this is still unlikely, but as the interface becomes more widely known, the risk will increase.

Mandatory email verification (#478) will add a barrier to entry for trolls, but not an impenetrable one. On the other hand, #508 will give trolls an additional weapon. Concluding, it must be easy and straightforward for a superuser to delete the content created by a troll, preferably by just following a link to the offending content in the frontend.

The features that currently exist in order to delete sources and annotations should suffice. We just need to make sure that superusers can use them even when they are not the owner. This means that

• the delete buttons in the frontend must be visible not only to the owner, but also to superusers, and
• DELETE endpoints on the backend must admit superusers (I suspect this is already the case).

Needless to say, superusers must also be able to ban trolls, but this is already taken care of by the Django admin pages.