Cherrypick csv upload changes (#5268)

[timifasubaa]

(cherry picked from commit bd24f85)

This PR cherrypicks changes to the CSV flow that fixes the schema issues people are facing

1. It makes all new CSV uploads internally managed tables and not externally managed tables.
2. It fixes the issue from before where the schema from the form field was not applied to the schema and caused the table to be uploaded to the default namespace.
3. It gives us the option of forcing all csvs into one namespaces should we decide to do so for PII control reasons

@airbnb/superset-fork-maintainers cc @paulbramsen

[codecov-io]

Codecov Report

Merging #115 into airbnb-master will increase coverage by 13.55%.
The diff coverage is 8.33%.

@@                Coverage Diff                 @@
##           airbnb-master     #115       +/-   ##
==================================================
+ Coverage           63.3%   76.85%   +13.55%     
==================================================
  Files                363       45      -318     
  Lines              22245     8893    -13352     
  Branches            2503        0     -2503     
==================================================
- Hits               14082     6835     -7247     
+ Misses              8151     2058     -6093     
+ Partials              12        0       -12

Impacted Files	Coverage Δ
superset/db_engine_specs.py	49.91% <0%> (-2.17%)	⬇️
superset/config.py	92.48% <100%> (+0.05%)	⬆️
superset/utils.py	87.09% <0%> (-0.97%)	⬇️
superset/models/core.py	84.92% <0%> (-0.34%)	⬇️
superset/views/core.py	73.37% <0%> (-0.22%)	⬇️
...sets/src/explore/components/ExploreChartHeader.jsx
superset/assets/src/components/Checkbox.jsx
...perset/assets/src/dashboard/util/getEmptyLayout.js
...ts/src/dashboard/util/getComponentWidthFromDrop.js
... and 314 more

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[paulbramsen]

LGTM! Thanks Timi.

[graceguo-supercat]

LGTM