Add events/tags when a validator becomes jailed or slashed

[fedekunze]

Summary

User story:

As a user using a client I'd like to get a notification when a specific validator gets slashed or becomes (un)jailed, so I can decide which staking actions to take right after those events

Problem Definition

Currently you can only query a txs using the action unjail tag, but there's currently no way to get an event when a validator gets slashed or becomes (un)jailed

on stake/keeper/slash.go there're some comments that say:
// TODO Return event(s), blocked on tendermint/tendermint#1803

Proposal

Add the necessary events or tags and make them available for clients

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned

[cwgoes]

Ref #2448

[rigelrozanski]

yeah we need a way to query from the tag system - tag querier?

[fedekunze]

closing via #4387