Bug 1011604 - [Messages] Update to use <gaia-header>

Well, since we use gaia-header now, we don't need ```REQUEST_ACTIVITY_MODE_CLASS_NAME``` anymore. Moreover I see that we can simplify all that close/back code a bit. I tried to do it by myself just to help out a bit [1]. What do you think? Also to make sure that all tests work we'll need to wait for [2] though. 

[1] https://github.com/azasypkin/gaia/commit/e3e131ddb8fc8d17ae27303998c2c0b561eb0cc4
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=1057996