feat(debug-ui): extend epoch info api

[Longarithm]

Allows to query arbitrary epoch by id in the epoch validators page.
The interface allows to either input id or scroll back and forth between epochs.

The main caveat is:

• current epoch must be indexed by ValidatorInfoIdentifier::BlockHash because it wasn't finalised yet and it is a sign that its stats are being updated online;
• for past epochs we must use ValidatorInfoIdentifier::EpochId because BlockInfo data used for block navigation may be GC-d.

Other small fixes:

• Support NotEnoughChunkEndorsements kickout reason
• Display kickout among the roles for the next epoch because that's where it actually comes in place. Displaying it for the current epoch is confusing.

See video preview here https://near.zulipchat.com/#narrow/channel/295558-core/topic/Optimistic.20Block.20Design/near/498834846

[codecov]

Codecov Report

Attention: Patch coverage is 3.70370% with 156 lines in your changes missing coverage. Please review.

Project coverage is 70.49%. Comparing base (8b3fc09) to head (ea3511c).

Files with missing lines	Patch %	Lines
chain/client/src/debug.rs	0.00%	131 Missing ⚠️
chain/jsonrpc/src/lib.rs	19.35%	25 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12891      +/-   ##
==========================================
- Coverage   70.54%   70.49%   -0.05%     
==========================================
  Files         851      851              
  Lines      174983   175049      +66     
  Branches   174983   175049      +66     
==========================================
- Hits       123433   123401      -32     
- Misses      46419    46513      +94     
- Partials     5131     5135       +4     

Flag	Coverage Δ
backward-compatibility	0.36% <0.00%> (-0.01%)	⬇️
db-migration	0.36% <0.00%> (-0.01%)	⬇️
genesis-check	1.42% <0.00%> (-0.01%)	⬇️
linux	70.33% <3.70%> (-0.04%)	⬇️
linux-nightly	70.14% <3.70%> (-0.04%)	⬇️
pytests	1.73% <0.00%> (-0.01%)	⬇️
sanity-checks	1.54% <0.00%> (-0.01%)	⬇️
unittests	70.32% <3.70%> (-0.05%)	⬇️
upgradability	0.36% <0.00%> (-0.01%)	⬇️

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