feat: Make upload path selectable #39

This works for me, but for a more public site, I think I'll also add a boolean attribute named "upload_allowed" and "writable" to Config.Library..

Something to allow you to configure which directories can receive new uploads, and which directories we consider writable for other purposes (like editing meta data in yml, creating new thumbnails, ...)

LGTM 👌

I think that'll be all from me for today. I'll be on holidays for the next two weeks.
Not sure if I will get around to do any coding.
see ya!

 		keys := make([]string, 0, len(a.Library.Paths))
 		for k := range a.Library.Paths {
 			keys = append(keys, k)
 		if _, exists := a.Library.Paths[targetLibraryPath]; exists {

feat: Make upload path selectable #39

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