Web UI (GUI) Pulse rebrand + minor usability improvements

I just updated the Web UI for the Pulse rebrand.

Unless I missed something:

* All textual occurrences of Syncthing now read Pulse
* We are now using the Pulse SVG logo instead of the Syncthing bitmap logos

Also:

* In app.js, the app is now called pulse instead of syncthing.
* I updated the content very slightly to include semantic labels for the sections (which will also help with accessibility). 
* I tweaked the typography to improve the visual hierarchy.
* I tweaked the colours for better consistency and to reduce the palette. (I would like to see a consistent palette in the future where colours are not used for decorative purposes and instead have consistent secondary semantics. We should also move away from relying on colour for primary semantics as this is an accessibility issue for people with colourblindness.)

I’ve tested the Setttings menu, responsiveness, adding a new device, adding a new folder, and the disconnection alert. Please let me know if you notice something I’ve missed.

It would be great to see this go into the next build so we can have a consistent end-user brand between the web site, forum, and the Web UI.