Bug 950074 - [Messages] The "you received a message" notice has a bad layout r=schung

For some reason the commited CSS was wrong.
Converted it to flex box so that it looks correct also on bigger screens (like
the Peak).
Merge pull request #14669 from julienw/950074-fix-new-message-notice

Bug 950074 - [Messages] The "you received a message" notice has a bad la...