rectify the whole environment variables situation, especially on nextjs side

[cedricschwyter]

Describe the bug
we have a mess with regards to environment variables, especially on the frontend.

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
we should probably not allow exposed environment variables to be inlined at build time in order to be able to specify them at runtime only.

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.