grant = async (capability, key) => {
    // Merge current keys with the new key
    const capabilities = new Set([...(await db.get(capability) || []), ...[key]])
    await db.put(capability, Array.from(capabilities.values()))
  }
Grants a capability to an identity, storing it to the access control
database.
@param {string} capability A capability (e.g. write).
@param {string} key An id of an identity.
@memberof module:AccessControllers.AccessControllers-OrbitDB
@instance