Enable Teradata

[livinm]

New DB engine spec for Teradata:

• LimitMethod should be WRAP_SQL since Teradata does not supporting "LIMIT" clause (TOP) - without this change sqllab does not work
• Timegrains for Teradata are added

[codecov-io]

Codecov Report

Merging #5870 into master will decrease coverage by 14.25%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master    #5870       +/-   ##
===========================================
- Coverage      78%   63.74%   -14.26%     
===========================================
  Files          46      374      +328     
  Lines        9314    23331    +14017     
  Branches        0     2607     +2607     
===========================================
+ Hits         7265    14873     +7608     
- Misses       2049     8445     +6396     
- Partials        0       13       +13

Impacted Files	Coverage Δ
superset/db_engine_specs.py	55.45% <100%> (+0.26%)	⬆️
...sets/src/explore/components/ExploreChartHeader.jsx	71.42% <0%> (ø)
superset/assets/src/components/Checkbox.jsx	100% <0%> (ø)
...perset/assets/src/dashboard/util/getEmptyLayout.js	0% <0%> (ø)
...ts/src/dashboard/util/getComponentWidthFromDrop.js	96.15% <0%> (ø)
...erset/assets/src/SqlLab/components/QuerySearch.jsx	69.36% <0%> (ø)
...rc/explore/components/controls/AnnotationLayer.jsx	22.92% <0%> (ø)
superset/assets/src/welcome/DashboardTable.jsx	81.81% <0%> (ø)
...erset/assets/src/dashboard/util/injectCustomCss.js	91.66% <0%> (ø)
...ets/src/dashboard/components/dnd/DragDroppable.jsx	97.91% <0%> (ø)
... and 319 more

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[mistercrunch]

Please also add an entry in the docs here:
https://github.com/apache/incubator-superset/blob/master/docs/installation.rst#database-dependencies

[livinm]

@mistercrunch added
see #5885