The build of the admin-ui adds some time to the entire build process.

We should only build when is needed, to achieve this the following is proposed:

• Split into a separate maven module
• Configure gh-actions to only build when there is a change in the related files
• When building, push the build code to a separate repo (notifications-frontend-admin-build)
• notifications-backend should download latest master from notifications-ui-admin-build and use it (maybe this step only happens in jenkins?)