Introduces a new COB type to store repository identity documents.
The reason for this change is to:
1. Simplify the code, as the identity document logic resembled a COB,
yet it had custom logic. This allows existing COB code to be used for
identities.
2. Make identity document update logic more flexible, since COB actions
can be added in the future.
3. Re-purpose existing tools around COBs to work on identities, eg. `rad
cob`.
4. Unify the concept of an identity change proposal, with regular identity
changes. This means we can remove the `id.proposal` COB in favor of
using the `id` COB itself.
Notes
-----
* Each repository has one Identity COB.
* The `Proposal` COB has been repurposed into the `Identity` COB.
* Identity documents are stored as *embeds* inside the Identity COB
actions.
* The action that contains new document versions is called `revision`,
just like the Patches COB.
* The namespaced `rad/id` ref is a symbolic reference to that Identity
COB.
* The canonical `rad/id` ref is a direct reference to the commit in the
Identity COB that contains the latest *accepted* revision of the
document.
* All commands for managing identities have been folded into `rad id`.
Hence `rad delegate` and `rad edit` are removed.
* The concept of "rebasing" an identity document is gone.
* The `rad id` output has been updated to match the style of other
commands.
* When a revision has enough signatures, it is automatically adopted as
the current identity, there is no longer the need for a "commit"
action.
* When an identity revision is proposed, and the current identity has a
threshold of `1`, that identity is automatically accepted due to the
above point.
* The idea of "verifying a peer's identity branch" no longer applies, as
COBs cannot be verified one history at a time.
* Since the root commit of the Identity COB does not have a "Resource"
to point to (it would have to point to itself, which is impossible),
we've made the resource id optional for COBs.
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