Clean up datalayer code for db migrations

Monal/Classes/DataLayer.m

@@ -93,19 +93,10 @@ -(id) init
 
     //checking db version and upgrading if necessary
     DDLogInfo(@"Database version check");
-
-    //needed for sqlite >= 3.26.0 (see https://sqlite.org/lang_altertable.html point 2)
-    [self.db executeNonQuery:@"PRAGMA legacy_alter_table=on;"];
-    [self.db executeNonQuery:@"PRAGMA foreign_keys=off;"];
 
     //do db upgrades and vacuum db afterwards
     if([DataLayerMigrations migrateDB:self.db withDataLayer:self])
         [self.db vacuum];
-
-    //turn foreign keys on again
-    //needed for sqlite >= 3.26.0 (see https://sqlite.org/lang_altertable.html point 2)
-    [self.db executeNonQuery:@"PRAGMA legacy_alter_table=off;"];
-    [self.db executeNonQuery:@"PRAGMA foreign_keys=on;"];
 
     DDLogInfo(@"Database version check completed");
 

Monal/Classes/DataLayerMigrations.m

@@ -65,6 +65,10 @@ +(BOOL) migrateDB:(MLSQLite*) db withDataLayer:(DataLayer*) dataLayer
     }];
 
     return [db boolWriteTransaction:^{
+        //needed for sqlite >= 3.26.0 (see https://sqlite.org/lang_altertable.html point 2)
+        [self.db executeNonQuery:@"PRAGMA legacy_alter_table=on;"];
+        [self.db executeNonQuery:@"PRAGMA foreign_keys=off;"];
+
         NSNumber* dbversion = [self readDBVersion:db];
         DDLogInfo(@"Got db version %@", dbversion);
 
@@ -1186,6 +1190,11 @@ FOREIGN KEY('account_id') REFERENCES 'account'('account_id') ON DELETE CASCADE \
             }
         }
 
+        //turn foreign keys on again
+        //needed for sqlite >= 3.26.0 (see https://sqlite.org/lang_altertable.html point 2)
+        [self.db executeNonQuery:@"PRAGMA legacy_alter_table=off;"];
+        [self.db executeNonQuery:@"PRAGMA foreign_keys=on;"];
+
         //check if db version changed and invalidate state, if so
         NSNumber* newdbversion = [self readDBVersion:db];
         if([dbversion isEqualToNumber:newdbversion] == NO)