Misc clean up

* app.js: Introduce site_owner['support_email'] var; this helps keep
site_owner['email'] separate and useful for other things
(i.e.: main contact).
* app.js: Disable res.locals.breadcrumbs var; this is no longer used.

* contact_success.jade: Reformat response message body a wee bit.
* email_html_template.jade: Use 'tel' and 'mailto' URL links for email,
phone number info fields; quicker-n-easier reply.

* site_footer.jade: Add main email addr to contact info section and
remove 'mailto' linking from copyright info section.

* error.jade: Show error stack only when not NULL; this prevents an
empty HR and codebox from showing up.
Clean up err handling in lib/form_helpers.js

We now handle message delivery errors (i.e.: SendGrid related) with
routing to the appropriate error 503 page with some basic help text
for the end-user.

* error.jade: Add error status code '503' for use in lib/form_helpers.js.
Bit of clean up && new sections

* README.md: Add some more info regarding alerts & logging levels.
* README.md: Initial sectioning of Heroku Deployment notes.
* README.md: Relocate 'Further Reading' & 'References' sections.