Gallery cover url

[WithoutPants]

• Adds a new gallery route /gallery/:id/cover which returns the cover image data, as an alternative to getting cover via graphql.
• Adds cover member to GalleryPathsType so that this URL is included in the returned paths.
• Adds a default gallery image for when the gallery has no images.
• Fixes issue with the gallery preview scrubber when gallery has no images
• Fixes issue with lightbox hanging when clicking on a gallery in the gallery wall when the gallery has no images
• Documents a known issue with the DLNA server where shutting the server down leaves remnant goroutines.