Add snap packaging option

[Komediruzecki]

Add snap packaging option

Add snap packaging with electron-builder
Specified removable-media as additional interface
Specified snap confinement as strict
Fix unused icon in NotePageToolbar

Users should add interface plug to be able to access removable media ("/mnt", "/media" mount points) via snap CLI:
snap connect <snap-name>:<available-plug-name>
For boostnote-next and this additional interface:
snap connect boostnote-next:removable-media

Snap Name should be registered in Snapcraft

• Via UbuntuOne developer account: https://snapcraft.io/docs/registering-your-app-name
• Valid names are boostnote, boostnote-next
• Invalid names: boostnote.next, boostnote_next
• Because of this, 'executableName' option sets the corrrect name in package.json and no further changes are needed for packaging.

For publishing new versions follow:
https://snapcraft.io/docs/releasing-your-app

snapcraft login
snapcraft upload --release=stable boostnote-next.snap

For developers, you can install it for testing via:
sudo snap install boostnote-next --dangerous --devmode // for devmode (unconfined environment/isolation)
sudo snap install boostnote-next --dangerous // for strict i.e. set in package.json confinment (for testing as users sees the app)

Later use the same snap connect commands to add removable-media interface plug or others (if available)

To run successfully installed snap:
where <snap-name> // where boostnote-next

Run with: /snap/bin/<snap-name>