Show when proposals were approved for display on the UI

[RichardRed0x]

"Submitted X days ago" is confusing as the proposal may have been in the moderation queue for some time. The second research proposal for example was submitted 4/5 days before it was approved. When it appears on the proposals site it says "submitted 5 days ago" and it looks like it has been ignored with 0 comments from the community.

The "submitted" time doesn't line up with anything in the user's experience (i.e. they may have checked the site yesterday and not seen a proposal that is visible today and was submitted "2 days ago"). I think it would be better to replace or add "published" or "approved" x time ago.

[lukebp]

Corresponding politeia issue: decred/politeia#618