Add a moderation tool

[voxpelli]

If someone finds an unwanted webmention then they should be able to remove it. Kind of a critical feature and without it a version 1.0.0 can't really be claimed to be achieved.

When manual moderation is possible, automatic one can also be added on top – such as spam-protection – but many such mechanisms needs to be possible to audit by users and therefore the manual interface has to come first.