Indicate when there are more attachments in remote posts

Currently, Mastodon only processes the first four attachments to a post. Processing more attachments has been proposed but with the caveat that Mastodon has to set an upper limit on attachments to process.

Proposed behavior: in the interim and beyond, Mastodon should at least indicate whether there are any more attachments, and at least include a link to view on the original page. Otherwise, users may think that there are only 4 attachments.