Auto-save notes when leaving (remove "Save" button from editor)

[orthecreedence]

The idea is you get rid of the "ARE YOU SURE YOU WANT TO LEAVE? REALLY? WHY NOT SAVE YOUR CHANGES?!?!" alert and just save automatically. Split off from #144.

[pipari]

Hi, just an ongoing +1 for this - it is the only main usability issue I have with turtl because I keep forgetting and use ctrl-S => switch to browser etc. => reboot and lose data. Autosave would also be great. Thanks for all of your work on this! I use the linux and android apps daily.