Create a Review pallet + UI

[siman]

Any account should be able to review any other space on Subsocial.
We can reuse Posts with a new extension: Review

Software Desing

pub struct AccountInSpace {
    account: AccountId,
    space: Option<SpaceId>
}

pub enum PostExtension {
    // ... all prev. variants here
    SpaceReview(/* Recipient */ AccountInSpace)
}

decl_storage! {
    pub ReviewIdsByReviewer: double_map (AccountId, Option<SpaceId>) => Vec<PostId>;
    pub ReviewIdsByAccountRecipient: map AccountId => Vec<PostId>;
    pub ReviewIdsBySpaceRecipient: map SpaceId => Vec<PostId>;
}

Prehistory:

Jack Platts (W3F):