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: []
  }