Add option to remove all external links from TPac

Add option to remove all external links from TPac

The goal is to make it so that public catalog terminals have less ways out
into the rest of the internet. Hiding browser UIs doesn't help if patrons
can just click through to eventually hit google.

parts/login/help.tt2 likely needs a less brute-forceish fix.

Signed-off-by: Thomas Berezansky <tsbere@mvlc.org>