Fix reveal

See merge request id2/software/software-project-templates/reveal.js-gfz-corporate-design-template!3

Resolve "local server does not start a server" and 'pages not packed correctly'

Closes #1 and #2

See merge request software/software-project-templates/reveal.js-gfz-corporate-design-template!2

Refactoring

See merge request software/software-project-templates/reveal.js-gfz-corporate-design-template!1

- changed path to open it directly in browser
- added script to build GFZ theme files
- added script to start live server
- adapted changes to .gitlab-ci.yml