More efficient flat file list format and various UX improvements (#3)

This is a major upgrade with assorted things included.

- Navigation flows improved, search appears in URL history, cleared when navigating to another folder
- More efficient file list format for faster loads
- Efficient updates, never re-send full root another time (except at connection)
- Large number of watching and filelist updates (inotify issues remain)
- File size coloring
- Fixed ZIP generation random glitches (thread race condition)
- Code refactoring, cleanup, typing fixes
- More tests

Reviewed-on: #3

frontend/src/assets/svg/scissors.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32"><path d="M27.84 22.16A7.15 7.15 0 0 0 22.9 20H22l-2-2 7.98-8c2-2 2-6 0-8L16 14 4.02 2c-2 2-2 6 0 8l8 8-2 2H9.1c-1.7 0-3.5.74-4.94 2.16-2.55 2.55-2.9 6.33-.77 8.45.9 1 2.2 1.4 3.5 1.4a7 7 0 0 0 4.9-2.1 6.86 6.86 0 0 0 2.1-5.8L16 22l2.04 2.05a6.87 6.87 0 0 0 2.1 5.8A7.2 7.2 0 0 0 25.07 32c1.34 0 2.6-.46 3.53-1.4 2.13-2.1 1.8-5.9-.77-8.44zm-16.8 4.26A4.95 4.95 0 0 1 8.44 29c-.5.22-1.02.33-1.5.33-.5 0-1.16-.1-1.67-.6a2.3 2.3 0 0 1-.6-1.64A4 4 0 0 1 5 25.5a3.9 3.9 0 0 1 1-1.53c.44-.46 1-.8 1.52-1.05.5-.23 1.03-.34 1.52-.34.46 0 1.13.1 1.64.6.5.5.6 1.2.6 1.65 0 .5-.1 1.02-.3 1.52zm4.96-5.6a2.83 2.83 0 1 1 0-5.67 2.83 2.83 0 0 1 0 5.68zm10.73 7.9c-.5.5-1.18.6-1.66.6-.5 0-1-.1-1.52-.32a4.92 4.92 0 0 1-2.9-4.08c0-.47.1-1.13.6-1.64.5-.5 1.2-.6 1.65-.6.5 0 1.02.1 1.52.32a5.08 5.08 0 0 1 2.6 2.58c.25.5.37 1.02.37 1.5 0 .47-.1 1.13-.6 1.64z"/></svg>