Add signals pallet

Resolves #353

Implements code such that:

- 80% of validators (by stake) must be in favor of a signal for the network to
  be
- 80% of networks (by stake) must be in favor of a signal for it to be locked
  in
- After a signal has been locked in for two weeks, the network halts

The intention is to:

1) Not allow validators to unilaterally declare new consensus rules.

No method of declaring new consensus rules is provided by this pallet. Solely a
way to deprecate the current rules, with a signaled for successor. All nodes
must then individually decide whether or not to download and run a new node
which has new rules, and if so, which rules.

2) Not place blobs on chain.

Even if they'd be reproducible, it's just a lot of data to chuck on the
blockchain.

substrate/signals/pallet/LICENSE

@@ -0,0 +1,15 @@
+AGPL-3.0-only license
+
+Copyright (c) 2023 Luke Parker
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License Version 3 as
+published by the Free Software Foundation.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public License
+along with this program. If not, see <http://www.gnu.org/licenses/>.