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/model/rwfolder.go

@@ -1110,7 +1110,7 @@ func (f *sendReceiveFolder) handleFile(file protocol.FileInfo, copyChan chan<- c
 		blocksSize = file.Size
 	}
 
-	if f.MinDiskFreePct > 0 {
+	if f.MinDiskFree.BaseValue() > 0 {
 		if free, err := osutil.DiskFreeBytes(f.dir); err == nil && free < blocksSize {
 			l.Warnf(`Folder "%s": insufficient disk space in %s for %s: have %.2f MiB, need %.2f MiB`, f.folderID, f.dir, file.Name, float64(free)/1024/1024, float64(blocksSize)/1024/1024)
 			f.newError(file.Name, errors.New("insufficient space"))