Add membership form via easyverein

[friep]

follow up to #585: after internal discussions and taking into consideration data security, we rely on an external provider to accept membership applications now.

This PR merely changes the links in the dropdown to redirect to the form provided by easyverein. needs to be reviewed by the board before merging.

TODO:

• [x] delete the reference to become_member.membership_application and the depending code (i get an error when i remove this in translations.js)

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| Name | Status | Preview | Comments | Updated (UTC) | | :--- | :----- | :------ | :------- | :------ | | **correlaid** | ⬜️ Ignored ([Inspect](https://vercel.com/correlaid/correlaid/4pb16TQFHrLAUpsfFj6PnErdg7LP)) | [Visit Preview](https://correlaid-git-membership-form-correlaid.vercel.app) | | Nov 8, 2023 7:24pm |

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