Added support for MySQL

* In packages/cli/src/Db.ts, conditional test of dbType replaced by a
switch;
* removeAll() function adapted to not cause an error using MySQL;
* Added the cross-env module in the "serve" script in the
packages/editor-ui/package.json file. This was done to ensure
compatibility between platforms when declaring environment variables.
Without it, Windows compilation would give an error, for example;
* .idea added to .gitignore (IntelliJ IDEA solutions);

packages/cli/src/databases/mysqldb/index.ts

@@ -0,0 +1,3 @@
+export * from './CredentialsEntity';
+export * from './ExecutionEntity';
+export * from './WorkflowEntity';