Feature request: Watermark support

[rolandixor]

Hi, I'd like to suggest a new feature that would really set Pixelfed apart:

It would be nice to be able to add watermarks to our profiles, and then select the watermark we'd like to use on upload. This way, you could (for instance) preserve your watermark (or, for instance, a creative commons label) even when uploading from mobile devices or the web interface on the go.