Fix vote weights of ranked members in the Society pallet (#2758)

This PR fixes a bug in the tally accrual of approvals/rejections for
Candidates and Defender. The issue happened because:
- The `match maybe_old` is reducing `weight` from the tally:
```
Some(Vote { approve: true, weight }) => tally.approvals.saturating_reduce(weight),
Some(Vote { approve: false, weight }) => tally.rejections.saturating_reduce(weight),
```
- But `match approval` is accruing only `1` to the tally:
```
true => tally.approvals.saturating_accrue(1),
false => tally.rejections.saturating_accrue(1),
```

This way, if a member is rank 1 his vote is going to have weight 1 when
accruing but weight 4 when reducing from the tally. For example, let's
say:
- There's a Candidate with 0 approvals and 12 rejections;
- A ranked Member votes against the Candidate;
- The tally changes to 0 approvals and 13 rejections (should be 16);
- The Member changes his vote to an approval;
- Now tally changes to 1 approvals and 9 rejections, removing the
accrued approvals from other Members;
- If the Member keeps changing his vote, it wipes the tally clean.

So this PR changes `match approval` to accrue `weight` instead of just
`1` and changes the tests:
- Fixes `challenges_work`. This test started failing after the fix. The
reason is that the test assumes that all Members have equal weights to
their votes, but Member 10 is ranked, so his vote should have weight 4
against the Defender. So instead of using Member 10, I added Member 50
of rank 0 to keep the same logic;
- Improves `votes_are_working`. Added some assertions to check if the
tally is correct even after a ranked Member changes his vote a couple
times;
- Fixes `waive_repay_works`. Unrelated to the bug, but this test was
yielding a false positive. The test is ranking up Member 20, but
asserting the rank of Member 10, which is already ranked up.

---------

Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: command-bot <>