gui, lib/config, lib/model: Allow absolute values for minimum disk free space (fixes #3307)

This deprecates the current minDiskFreePct setting and introduces
minDiskFree. The latter is, in it's serialized form, a string with a
unit. We accept percentages ("2.35%") and absolute values ("250 k", "12.5
Gi"). Common suffixes are understood. The config editor lets the user
enter the string, and validates it.

We still default to "1 %", but the user can change that to an absolute
value at will.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4087
LGTM: AudriusButkevicius, imsodin

gui/default/syncthing/core/syncthingController.js

@@ -62,7 +62,7 @@ angular.module('syncthing.core')
                 selectedDevices: {},
                 type: "readwrite",
                 rescanIntervalS: 60,
-                minDiskFreePct: 1,
+                minDiskFree: {value: 1, unit: "%"},
                 maxConflicts: 10,
                 fsync: true,
                 order: "random",