Add permalinks to changeset comments

[gy-mate]

Fixes #2796

[HolgerJeromin]

Suggested change

	:time_ago => link_to(friendly_date_ago(comment.created_at), "https://www.openstreetmap.org/changeset/#{@changeset.id}##{comment.id}"),
	:time_ago => link_to(friendly_date_ago(comment.created_at), "/changeset/#{@changeset.id}##{comment.id}"),

But there are probably more rail-ish ways to create that url....

[gravitystorm]

You can generally get the path to an object using *_path helpers, e.g. changeset_path. So changeset_path(changeset) will link to the changeset. If you need an anchor (i.e. a #something in the url) then have a look through the views for other places that use the word "anchor", like for diary entry comments.

You will also find that in some circumstances you can simplify the link_to("foo", changeset_path(changeset)) further, since (most of the time) rails knows how to build a path for an object, e.g. link_to("foo", changeset)`. You can check other places in the existing codebase to learn what works.

[AntonKhorev]

There was a previous attempt #4134. It had the advantage of NOT adding link to the time, in case clicking the time is supposed to do something else like #4361.