Create script for updating an existing app

When you generated an app via tei-publisher, you may want to update its general code libraries and styles whenever tei-publisher has a new release. Suggested approach:

1. Create an update page which lists all apps available in the eXistdb instance, e.g. in a select. To get all installed apps, call repo:list(), then find out where they are installed in the db and check if they were created from tei-publisher.
2. When the user requests an update, copy everything in
   • templates/basic/modules/lib
   • templates/basic/resources/css, except _content.less

To make clear which files a user can change, we should maybe set them to read-only in the post-install.xql of the generated app.

Related #66 (closed), #62 (closed)