Improve visibility of slashes

[traumschule]

The validators page (#4722) shows Slashed in validator details (#4327). Since p-js doesn't expose this information very well can do a better job to give operators and nominators insights\

1. When past slashes happened to also help nominators judging the reliability of a node.
2. If known show hints why slashes were applied (reasons are listed here and be linked in a tooltip.
3. When multiple slashes were applied to several nodes in an era, because one criterion to be slashed is: Slashing starts when 10% of the active validators set are offline and increases in a linear manner until 44% of the validator set is offline; at this point, the slash is held at 7%.

Steps

• When Slashed is non-zero add a tooltip with information from staking.staking.slashingSpans.prior listing the era. Add this link for reference.
• Include a list of affected nominators.
• When other nodes were offline in the same era include also affected with a list of those validators. See also offences.concurrentReportsIndex: Vec<H256> A vector of reports of the same kind that happened at the same time slot.
• Make it clear of there were non-zero slashes which were actual slashes or "just" being booted for being offline to give nominators more context of what might have happened.
• At some point a separate tab dedicated to slashes would make sense for CM and others to better assess such chain events. There also is a SlashJournalEntry exposes more relevant information. (note that i haven't found this option in our chain state yet, please add if you know details):

"SlashJournalEntry": {
        "who": "AccountId",
        "amount": "Balance",
        "ownSlash": "Balance"
      },

Details

For context: apparently there were multiple slashing spans during the early mainnet when the number of nodes was much less:

staking.slashingSpans
{
  spanIndex: 6
  lastStart: 624
  lastNonzeroSlash: 0
  prior: [
    4
    2
    11
    1
    44
    113
  ]
}

It can help to include information about offences.reports which reflect when a validator was caught offline without prior chilling. As i understand singular im-offline reports are without penalty and only contain the list of nominators like so:

offences.reports(0x4fa0b519bd241cc4233ffb729d619209b4269252d697340efb0b8e61d3f5aa95)
{
  offender: [
    j4RLnWh3DWgc9u4CMprqxfBhq3kthXhvZDmnpjEtETFVm446D
    {
      total: 23,395,766,779,102,740
      own: 2,050,000,000,000,000
      others: [
        {
          who: j4WimuwF1N3nqff211ifeGSW8q5DW8xWpsjaNf9Feds1MDo5B
          value: 2,008,697,142,401,933
        }
        {
          who: j4UduAvo4BxvuWXCfaddD1C3MtAJtNVpqWWfMMiKk1q3Hcdx6
          value: 20,543,932,750,929
        }
        {
          who: j4TN2fgVJjsATiojEnJdF4ofKx2UfGnDYEnRdrzQ1SSiAkC9o
          value: 19,316,525,703,949,878
        }
      ]
    }
  ]
  reporters: []
}