Create env variable that enables us to deploy just the static pages from the dev branch

[positiveimpact]

Right now we're deploying pre-launch-website branch to climateconnect.earth. On that branch all pages besides a few necessary static pages are commented out.
The problem with this setup is, that we need to always manually comment out new pages, if we merge in develop and this might lead to manual errors. Therefore we should create an env variable PRE_LAUNCH.

In PRE_LAUNCH mode, if you access the main page should serve the /about page. Also none of the links in the header besides the logo should be shown. The only other pages, that should be accessible are /impressum and /faq. The footer can be shown as usual.
This depends on #49