dashboard: update to v2020.09.08.1 and add enable-experimental config (#2926)

[ti-srebot]

cherry-pick #2926 to release-4.0

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What problem does this PR solve?

• dashboard: update to v2020.09.08.1
• dashboard: add enable-experimental config

Check List

Tests

• Unit test

Code changes

• Has configuration change

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Release note

• dashboard: update to v2020.09.08.1
• dashboard: add enable-experimental config

[HunDunDM]

LGTM

[lhy1024]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

@ti-srebot merge failed.

[lhy1024]

/run-integration-lightning-test

[lhy1024]

/run-integration-lightning-test

[codecov]

Codecov Report

Merging #2928 into release-4.0 will increase coverage by 0.16%.
The diff coverage is 66.92%.

@@               Coverage Diff               @@
##           release-4.0    #2928      +/-   ##
===============================================
+ Coverage        77.16%   77.33%   +0.16%     
===============================================
  Files              204      209       +5     
  Lines            21940    22866     +926     
===============================================
+ Hits             16931    17684     +753     
- Misses            3724     3840     +116     
- Partials          1285     1342      +57     

Impacted Files	Coverage Δ
client/metrics.go	100.00% <ø> (ø)
pkg/apiutil/apiutil.go	67.34% <ø> (ø)
pkg/cache/cache.go	0.00% <ø> (ø)
pkg/cache/fifo.go	100.00% <ø> (ø)
pkg/cache/lru.go	80.35% <ø> (ø)
pkg/cache/two_queue.go	66.66% <ø> (ø)
pkg/codec/codec.go	78.57% <ø> (ø)
pkg/component/manager.go	81.01% <ø> (ø)
pkg/dashboard/adapter/redirector.go	94.23% <ø> (ø)
pkg/dashboard/keyvisual/input/core.go	33.33% <ø> (ø)
... and 208 more

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