SImplify default values for frontend kubernetes deployment

[BacLuc]

Currently our defaults for the frontend are a little distributed.
We have defaults for the development, but they don't apply for the kubernetes deployment.
(But we could use the same defaults).
We set values in our github action to the same value which would be the default for the development.
(e.g.

termsOfServiceLinkTemplate: # 'https://ecamp3.ch/{lang}/tos'
newsLink: # 'https://ecamp3.ch/blog'
helpLink: # 'https://ecamp3.ch/faq'

(https://github.com/ecamp/ecamp3/blob/8bfdbee9f7a52b64d21a05e669240b40e85a4976/.helm/ecamp3/values.yaml#L11,L13)

Proposal: We use one set of defaults.
If the default for kubernetes needs to be different, we set it to a value in values.yaml.
The json in the frontend configmap only contains the values we want to overwrite. (and maybe just use a tojson function for all the values).
In the github action, we only set the helm values if the env value or secret is not null, else the usual chain of defaults applies.

[BacLuc]

Maybe improved with:

• Rewrite deploy ecamp3 #8025
• frontend: use vite's mechanism for env variables to fill window.envir… #3268