(bug 5191) can make entries with slug URLs sticky

Previously, you could only use a ditemid or a URL with ditemid to set
a sticky entry; putting in a URL with a slug would lead to an error
message.

I modified sub sticky_entry in cgi-bin/LJ/User.pm so that it would accept
ditemids, URLs with ditemids, and URLs with slugs. sticky_entry still
validates the entry and uses its ditemid to set the sticky property.

Testing:
Used the free individual account test_free on my 'hack

Strings that set the corresponding entry as sticky:
* "http://test-free.quartzpebble.hack.dreamwidth.net/1202.html"
* "http://test-free.quartzpebble.hack.dreamwidth.net/2014/03/25/slugbug.html" (public entry)
* "http://test-free.quartzpebble.hack.dreamwidth.net/2014/03/24/friendly-slug.html" (access-locked entry)
* "http://test-free.quartzpebble.hack.dreamwidth.net/2014/03/25/private-slug.html" (private entry)
* "test-free.quartzpebble.hack.dreamwidth.net/2014/03/25/private-slug.html"
* "test-free.quartzpebble.hack.dreamwidth.net/2014/03/21/private-slug.html" <-- note date doesn't matter
* setting "" or " " still removes the current sticky entry

Strings that resulted in an error message ("Invalid Dreamwidth entry ID or URL entered."):
* "slugbug"
* "private-slug"
* ";ouasenouth"
* '!@$%(*#&%#"@'
* "http://test-free.quartzpebble.hack.dreamwidth.net/2014/03/25/private slug.html"
* "http://test-free.quartzpebble.hack.dreamwidth.net/2014/03/25/private-slug"
using non-capturing groups for slug-finding regex
Merge pull request #657 from fhocutt/bug5191-slug-stickyentry

(bug 5191) can make entries with slug URLs sticky