[#928] Remove permalink TODO from new entry page

The slug lets people generate a link for their entry before posting --
better than the ditemid.html form, which we'd only find after posting /
on edit anyway.
[#928] Use LJ::Entry methods to grab the url

* for individual entry loading

* fixes entry urls with slugs

*  uses methods on LJ::Entry to query, e.g., poster, instead of doing
  a raw SQL query
[#928] Use LJ::Entry->url when generating feeds

* so that we automatically show the slug when necessary

* ...except for the id/guid-type elements
[#928] Switch from LJ::item_link to $entry->url

* ...except for LJ::Protocol::getevents and memories.bml (loads in bulk
  via sql, possible performance hit)

* also not in update.bml because there's no interface to add a slug
  there

* additional tweak: remove the style=mine from the RPC cut expander; we
  don't get passed the style arguments

Fixes #928.
Merge pull request #929 from afuna/entry-link-bug-928

Use $entry->url where possible so that the slug url can be used