Display summary of attachments in description, and mark up content
warning if present, e.g.:

    Attached: 3 images · Content warning: Dota 2

When text is not supposed to be hidden, it looks more like:

    Attached: 3 images

    Here is the text of the toot

With #6817, multilinguagility should be assured...

- Add missing meta description to profiles
- Add canonical rel link to landing page
- Remove linebreaks from title tags
- Add username to profile title
- Add toots/following/followers to profile description tags
- Add next/prev rel links to profiles
- Do not index follower/following variants of profiles

- Return all images as og:image
- Return videos as og:image (preview) and og:video
- Return profile:username on profiles

* add og:url to stream_item view

* add og:url and text to about/show

* add og:url to profile, followers and follows pages

* Add meta tags for OGP to following & follow page

* simplify render helper

* Follow up #2133

* og:type is only accounts/show

* revert accounts/{following,followers}