Added notification logic to Notifier; dispatch notifier_enabled when toggled
so the toolbar can be shown/hidden and the button text can be kept in sync.
Add MatrixToolbar back into MatrixChat for notification nagging.
* sans-serif font
* use display: table for the message list so that everything lines up nicely
* switch to flexbox layout rather than position absolutes to avoid assuming a full screen UI and improve maintainability (and better match atomify)
* neutralise the colouring and styling of highlighted rooms in the recents list
* switch from ul + li's to a series of divs in some places where the li's are achieving nothing but adding more complexity to the DOM
* add mx_MessageTileType as a generic css base class for all message tiles
