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

lib/config/config_test.go

@@ -55,7 +55,7 @@ func TestDefaultValues(t *testing.T) {
 		CacheIgnoredFiles:       false,
 		ProgressUpdateIntervalS: 5,
 		LimitBandwidthInLan:     false,
-		MinHomeDiskFreePct:      1,
+		MinHomeDiskFree:         Size{1, "%"},
 		URURL:                   "https://data.syncthing.net/newdata",
 		URInitialDelayS:         1800,
 		URPostInsecurely:        false,
@@ -110,7 +110,7 @@ func TestDeviceConfig(t *testing.T) {
 				Pullers:         0,
 				Hashers:         0,
 				AutoNormalize:   true,
-				MinDiskFreePct:  1,
+				MinDiskFree:     Size{1, "%"},
 				MaxConflicts:    -1,
 				Fsync:           true,
 				Versioning: VersioningConfiguration{
@@ -201,7 +201,7 @@ func TestOverriddenValues(t *testing.T) {
 		CacheIgnoredFiles:       true,
 		ProgressUpdateIntervalS: 10,
 		LimitBandwidthInLan:     true,
-		MinHomeDiskFreePct:      5.2,
+		MinHomeDiskFree:         Size{5.2, "%"},
 		URURL:                   "https://localhost/newdata",
 		URInitialDelayS:         800,
 		URPostInsecurely:        true,