Prevent KeeShare from merging database custom data

This issue previously caused parent databases to be marked as modified on unlock. This was because of the new protections against byte-by-byte side channel attacks adds a randomized string to the database custom data. We should never be merging database custom data with keeshare or imports since we are merging groups only.

Also prevent overwrite of auto-generated custom data fields, Last Modified and Random Slug.

tests/TestMerge.h

@@ -30,6 +30,7 @@ private slots:
     void cleanup();
     void testMergeIntoNew();
     void testMergeNoChanges();
+    void testMergeCustomData();
     void testResolveConflictNewer();
     void testResolveConflictExisting();
     void testResolveGroupConflictOlder();