[SIP-4] replace dashboard ajax calls with SupersetClient

[williaster]

This PR is one of a few PRs that implement the final step 4) discussed in #5772, to refactor just dashboard-specific ajax calls (not including charts) for easier review

Note that the new @superset-ui/core dep + setup for SupersetClient is duplicated across all ajax PRs:

• dashboard (this PR)
• explore [SIP-4] replace explorer ajax calls with SupersetClient #5869
• charts [SIP-4] replace chart ajax calls with SupersetClient #5875
• sql lab [SIP-4] replace SQL Lab ajax calls with SupersetClient #5896
• logging + misc

@kristw @mistercrunch @graceguo-supercat @michellethomas @conglei

[codecov-io]

Codecov Report

Merging #5854 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5854   +/-   ##
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  Coverage   77.28%   77.28%           
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  Files          47       47           
  Lines        9332     9332           
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  Hits         7212     7212           
  Misses       2120     2120

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[graceguo-supercat]

I do have some concerns for this PR:

• It will replace jquery.ajax with a new toolkit (based on fetch, added helper and wrapper) in all Superset.
  Is there any existed Superset dependencies that depends on jquery?

• This toolkit lives outside Superset codebase.
  Given it didn't have any mature customers yet, I feel we (Superset developers) may hit into its bugs or add improvement pretty often. Superset frontend is ajax-heavy. For the long term, when we develop some new features (like improve logging or batch-fetch etc, maybe?), I need to consider should i add this additional function in Superset or in the toolkit code base?
  When we need a little change on FAB, I need to fix it in another codebase, tested it in another codebase with many context i am not familiar, wait it build and generate a new version, then i can use it in Superset. This toolkit will be much more friendly than FAB, but the repo is different :)

• Is it possible that make the new toolkit inside Superset, but generate standalone modules, to share with embeddable chart?

I guess all above are somewhat discussed in the meeting. If all of our major contributors (especially frontend contributors) accept inconvenience, i can accept it too :)

@mistercrunch @betodealmeida @hughhhh @xtinec @JamshedRahman @jeffreythewang @fabianmenges @michellethomas @williaster @kristw @conglei

[williaster]

good comments, @graceguo-supercat. Several good questions in there, I added my response to the actual SIP-4 here.

[graceguo-supercat]

LGTM