Allow users to edit the manifesto

[julienbrg]

Should be plain markdown in the mission statement input field.

[julienbrg]

Example in the current version:

• https://www.tally.xyz/gov/helpers-all-stars
• https://goerli.etherscan.io/address/0x58b4E02c35Ca361B96ca6A319Ae1106356382211#readContract
• https://bafybeify4byk6uixfoj7ooga4dltck5gyc3zofq6ept7iz64otwcatu24m.ipfs.w3s.link/manifesto.md