Bug 939729 - With httpd.js we sometimes don't get the most recent version of the file r=vingtetun

This patch:
* makes all files expire immediately, forcing a revalidation by Firefox
* implements conditional requests with Last-Modified-Since
Bug 939771 - xfail test_setup_basic_gmail.py
Merge pull request #13798 from bebef1987/xfail_email

Bug 939771 - xfail test_setup_basic_gmail.py
Merge pull request #13793 from julienw/add-expiration-and-conditional-content-logic-in-httpd-js

Bug 939729 - With httpd.js we sometimes don't get the most recent versio...
Bug 939722 - Xfail test_cost_control_data_alert_mobile
Merge pull request #13800 from bebef1987/xfail_cost

Bug 939722 - Xfail test_cost_control_data_alert_mobile
Bug 936639 - [Messages][Drafts] Create Drafts object + tests