Change attachment photo image alt texts to the title

Currently the alt texts for the attachment photos are defined by the photo's description as defined by: https://github.com/decidim/decidim/blob/372d1114e364ac1a0553fcf758514487b49cb2d7/decidim-core/app/views/decidim/application/_photos.html.erb#L9 I suggest changing this to the photo titles instead because for example the proposal attachments do not have the alt texts defined currently because they don't have any description. This would improve: - The meta information of the images for them to be better discoverable by search engines - Usability for blind users