[bug 914122] Don't use letter-spacing on grid icons, as it results in poor glyph spacing

See bug 914122 comment 40. Using negative letter-spacing gives poor-looking text, because the glyph positions are rounded to whole pixels and therefore fine adjustments to the individual positions cannot be accurately realized; instead, we get a few abrupt, ugly whole-pixel shifts at "random" places within the string.
Bug 938931 - [FTU] Fix checkbox regressions from bug #914892
Bug 939149 - [Action Icons - FM Radio] Remove shadows from action icons
Bug 939148 - [Action Icons - Calendar] Remove shadows from action icons
Bug 930544 - Allow installing applications on several empty screens
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 930104 - Remove localId=0 hack for the system app on firefox and correctly set permission for any app loaded in Firefox r=vingtetun
Revert "Merge pull request #13769 from timdream/lockscreen-unlock-transition"

This reverts commit 8b3338df8cb0144f2c2826b4b2d5c1841acdd038, reversing
changes made to 28e7dd59cf5b8ad9b197c59fbf92603e934e0474.
Bug 937630 - All you need is ... smooth unlock transition
Merge pull request #13714 from jfkthame/patch-2

Bug 914122: Don't use letter-spacing on grid icons, as it results in po...
Merge pull request #13781 from pivanov/bug-939149

Bug 939149 - [Action Icons - FM Radio] Remove shadows from action icons
Bug 933561 - App does not come to foreground if user quickly toggles between home screen and app
Bug 939720 - Failure in test_ftu_skip_tour
Merge pull request #13795 from timdream/lockscreen-unlock-transition2

Bug 937630 - All you need is ... smooth unlock transition, r=vingtetun
Merge pull request #13796 from viorelaioia/bug_939720

Bug 939720 - Failure in test_ftu_skip_tour
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