comments: Update signature and settings.

[lukebp]

Update signature

The comment signature needs to include the ExtraData and ExtraDataHint fields.

The New and Edit commands of both the politeiawww and plugin APIs need to be updated to reflect this change.

This is also a good time to make the same documentation improvements that we've been making in other areas. The existing documentation looks something like this:

// Signature is the client signature of the:
// State + Token + ParentID + Comment

Let's make it more detailed. Example:

// Comment represent a record comment.
//
// PublicKey is the user's public key that is used to verify the signature.
//
// Signature is the user signature of the:
// State + Token + ParentID + Comment + ExtraData + ExtraDataHint
//
// Receipt is the server signature of the user signature.
//
// The PublicKey, Signature, and Receipt are all hex encoded and use the
// ed25519 signature scheme.
...

Update plugin setting

An AllowExtraData plugin setting needs to be added to the comments plugin that defaults to false. We don't want users to be able to submit arbitrary data in the ExtraData and ExtraDataHint fields. The purpose of these fields is to allow application specific functionality to be layered on top of the comments plugin. An application should add in validation for these fields if it enables this setting.