Create a general messaging ui component

This will have the side effect of removing everyone's alpha message preference, but I'm not sad about this.
Move the whole alpha message out of the module
Fix the exit to desktop link
Messages should overwrite each other, not append
Rename the releaseMessage fragment to avoid confusion
Merge branch 'master' of github.com:guardian/frontend into messaging

Conflicts:
	common/app/assets/javascripts/bootstraps/common.js
Auto-signin now uses the Message API
Rename seen() to remember()
Message now has some jasmine tests
Remove the `this` references to bonzo objects

I think it actually makes things more obscure, so I removed them.
Add a concept of importance to the messages

Only messages marked as important can overwrite other messages.

This still isn't particularly great UX, but it will stop people mistakenly overwriting
each others messages.
Formatting
Merge pull request #2335 from guardian/messaging

Genericise the UI flash messaging