Bug 1091486 - [SMS] Switch to MMS when entering a email recipient thread

Just wondering, looks like (at least logically while Composer.js isn't always good at logic :), so maybe I'm missing something) we just need suppress type conversion banner for ```Compose.clear``` called inside ```this.cleanFields``` + exception when we enter email thread from thread list (ThreadUi.beforeEnterThread->ThreadUI.initializeRendering->ThreadUi.cleanFields). If so, do you think it's possible to use ```disable\enableConvertNoticeBanner``` only inside ```cleanFields```?