prevent running migrations if is_fresh_install

+ fix: sqlite3.ProgrammingError: Cannot operate on a closed cursor on ProcessAlbumColors()
+ move processing artist images from periodic_scans to Populate
+ bump hash string limit from 7 to 10
+ add last_mod property to database
+ fix: TypeError: '<' not supported between instances of 'int' and 'str' on album page

app/migrations/__preinit/__init__.py

@@ -28,6 +28,10 @@ def run_preinit_migrations():
     except OperationalError:
         userdb_version = 0
 
+    # No migrations to run
+    if userdb_version == 0:
+        return
+
     for migration in all_preinits:
         if migration.version > userdb_version:
             log.warn("Running new pre-init migration: %s", migration.name)