Set X-Frame-Options header to all responses

https://bugzilla.mozilla.org/show_bug.cgi?id=922043
Use gravatar over HTTPS

https://bugzilla.mozilla.org/show_bug.cgi?id=922078
Use is_safe_url() instead of home-grown check

https://bugzilla.mozilla.org/show_bug.cgi?id=922539
Better reuse of selectors
Upgraded Magnific Popup plugin to v0.9.8
Added data API for linking to contact forms

Fixes #42.
Accept `sfields` to be a comma-separated string of fields

This aligns with the query parameters accepted in the editor and allows
using both comma-separated and multiple parameter forms.

Fixes #46.
Cleanup: coding style fixes
Simplified code