Bug 1367604 - BMO extension to apply security policies to Phabricator revisions as...

Bug 1367604 - BMO extension to apply security policies to Phabricator revisions as needed

* - Updated based on dylans review
- Fixed custom policy to instead allow projects and subscribers and then
  add BMO roles to the subscriber list
- Some other bug fixes

* fix lifetime of phabricator_url_re()

Instead of passing the value (which depends on runtime configuration)
pass in a reference.
Also edit extensions/BMO/Extension.pm to allow %autodetect_attach_urls
regex option to be a callback instead of just a plain regexp ref.

* - Fixed regex in BMO extension to detect phabricator attachments
- Use request_cache for useragent handle in Util.pm