Update dispute participation on active leaves update

[BradleyOlson64]

Here are the goals that should be addressed by the PR so far:

• Bump dispute from best effort to priority as soon as it learns about the disputed block inclusion
• Add dispute to best effort for freshly backed candidate if it isn’t already included
• Check whether the freshly included block was causing an increase in spam slots for validators and decrement spam slots for each validator effected
• Change where spam slots are increased, so that they aren’t so long as the block is backed. (previously the block needed to be included)
• Check whether a freshly backed block was causing an increase in spam slots for validators and decrement spam slots for each validator effected
• Make sure not to move disputes from priority to best effort on backing events

Here's what remains to be done:

• Possibly change spam slots to be indexed by candidate hash alone, since those are already unique
• Incorporate Robert's hints about getting session index (current way of acquiring session is wrong)
• Add tests!
• Change documentation to reflect these changes

Fixes #5875

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: test-linux-stable
Logs: https://gitlab.parity.io/parity/mirrors/polkadot/-/jobs/2155964

[eskimor]

Looks really good. Great work, Bradley!