Proposal signing

[bastihav]

Description

Draft until certificates is merged, as this already uses certificate's implementation

Implements part of #594

Reason for this PR

• We want to sign proposals in the frontend

Changes in this PR

• implement signing and verification of signatures for proposals

Type of change (remove all that don't apply)

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

Manually tested with Matthias.

• Manually tested all use cases this could alter

Test Configuration:

• OS: Windows
• Browser: Chrome

Checklist: (remove all that don't apply)

• I added corresponding unit tests (if API code)
• I have performed a self-review of my own code
• My changes generate no new linting warnings or console warnings
• I have updated the CHANGELOG.md to include any changes made in this PR (add WIP to the top, if there is none already)

[tobias-wiese]

looks good