Support for the `text` property of the `photo` entity on the frontend

The protocol does support a text property for the photo entity: https://diaspora.github.io/diaspora_federation/entities/photo.html

To improve the accessibility it would be good if the frontend would add support for it. By now there doesn't seem to be a possibility to add a text when uploaded a photo. If not done, of course also the processing and displaying of this alternate text should be supported.