[nnbd_migration] Change edit links to no longer be generic "post links"

Originally, edit links were implemented as generic "post links," that
had no special handling except to fire a post request.

At this point, this is no longer the case. We at least want to
specialize the error message, and we have TODOs to do more
specialization (targeted reloading of the UI).

Note that 'edit-link' is already a class, it refers to the edits the
migration tool suggests as the migration. Therefore I used the class
'add-hint-link' instead of 'edit-link.'

Lastly, post-links were styled like buttons. But they are not the only
links that should be styled as buttons now, so just use that class
instead of having special styling.

Change-Id: I8241a5ec24e6a290b94a75327c06257abdfc011b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/144660
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Mike Fairhurst <mfairhurst@google.com>