v0.24.0-beta release

This is testing release only!

This is fairly big release. Important highlights:

• (experimental) new note type "relation map" for visualizing relations between notes
• new concept of "mirror" relations - relation can require another relation in the opposite direction
• we now parse out note content "links" - e.g. hyperlink from one note to another, image reference or inclusion of note in the relation map. These links are also visible in the relation map.
• images are now represented as "image" notes, i.e. they are placed into a tree structure
  • there's a migration script which puts the image under every note in which it is placed
• jQuery UI removed completely from the application, using mostly Bootstrap (upgraded to v4) to replace its functionality
• consolidated icon usage, moved to Jam icons (previously used mix of feather icons, custom icons, glyphicons and jQuery UI icons). Jam icons look remarkably similar to feather icons so overall look stays very similar
• Experimental Evernote import

Because of changes in image representation, synchronization protocol changed and all instances in sync cluster needs to be upgraded at the same time.