Enforce spaces around magic links (RFC, PMID, and ISBN).

Ensure that there is a \b boundary before and after RFC, PMID, and ISBN
links.  (Previously we enforced \b boundaries only before free external
links and after ISBN links.)  Consistency is a good thing!

In addition:
* \b is not a PHP escape sequence, so you don't need to write \\b inside
  a string.
* \b before the numeric part of an ISBN is pointless: by the structure
  of the regexp there will always be a space on the left and a word
  character (a digit) on the right.

Bug: 65278
Change-Id: Ic315b988091a5c7530a8285b9249804db72e55db