Generate tag overview, add file action

CHDataManagement/Model/Tag.swift

@@ -26,12 +26,19 @@ final class Tag: Item {
         super.init(content: content, id: id)
     }
 
-    var linkName: String {
-        id.lowercased().replacingOccurrences(of: " ", with: "-")
+    func isValid(id: String) -> Bool {
+        content.isValidIdForTagOrPageOrPost(id) &&
+        content.isNewIdForTag(id)
     }
 
-    var url: String {
-        "/tags/\(linkName).html"
+    @discardableResult
+    func update(id newId: String) -> Bool {
+        guard content.storage.move(tag: id, to: newId) else {
+            print("Failed to move files of tag \(id)")
+            return false
+        }
+        id = newId
+        return true
     }
 
     // MARK: Paths