Add description and nice photo when sharing

It will use the first sentence of the notes as description

It will also use the first photo as default thumbnail, if present. Otherwise
it will use one of the stock photos.
og:image links multiple photos

If multiple photos are present, several og:image tags are generated, so the user
is able to cycle the large high resolution ones when picking a preview, instead of
just their thumbnails.
Remove tags on deletion

There was a bug, when deleting a dive, the tag index would still
point to the unexisting dive.

This commits makes it schedule a task to remove the dive from the
tags.