Implement Staking Reward Eras basics (#1589)

Implement the basic functionality of tracking and rotating Reward Era. Closes #1567
Does not include anything to do with the Reward Pool.

- [x] Chain spec updated
- [x] Design doc(s) updated
- [x] Tests added

pallets/capacity/src/tests/eras_tests.rs

@@ -0,0 +1,25 @@
+use super::mock::*;
+use crate::{
+	tests::testing_utils::{run_to_block, system_run_to_block},
+	Config, CurrentEraInfo, Error, Event, RewardEraInfo,
+};
+
+use frame_support::traits::Get;
+
+#[test]
+fn start_new_era_if_needed() {
+	new_test_ext().execute_with(|| {
+		CurrentEraInfo::<Test>::set(RewardEraInfo { current_era: 1, era_start: 0 });
+		system_run_to_block(9);
+		run_to_block(10);
+		let mut current_era_info = CurrentEraInfo::<Test>::get();
+		assert_eq!(current_era_info.current_era, 2u32);
+		assert_eq!(current_era_info.era_start, 10u32);
+
+		system_run_to_block(19);
+		run_to_block(20);
+		current_era_info = CurrentEraInfo::<Test>::get();
+		assert_eq!(current_era_info.current_era, 3u32);
+		assert_eq!(current_era_info.era_start, 20u32);
+	})
+}