[sqllab] Include visual feedback that SQL Lab query is limited

[jeffreythewang]

As an extension to #5392, I think it makes sense to include visual feedback when the query has been limited on the backend.

@timifasubaa @villebro

[codecov-io]

Codecov Report

Merging #5774 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5774      +/-   ##
==========================================
+ Coverage   77.55%   77.55%   +<.01%     
==========================================
  Files          47       47              
  Lines        9485     9486       +1     
==========================================
+ Hits         7356     7357       +1     
  Misses       2129     2129

Impacted Files	Coverage Δ
superset/sql_lab.py	71.92% <100%> (+0.16%)	⬆️

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[villebro]

@jeffreythewang #5392 needs some work as it introduced a regression (see discussion), probably best to wait for a fix for that first and then take a new look at this PR.

[jeffreythewang]

@villebro @timifasubaa A bit late, but I just rebased this after seeing that the regression mentioned in #5392 was resolved.

[stale]

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