Improve Unicode support for MSSQL

[villebro]

The upcoming SQLAlchemy 1.3 release introduces N-prefixed Unicode string handling on MSSQL (see sqlalchemy/sqlalchemy@e1b299d ). This PR introduces support for that feature and has been tested on master branch of SQLAlchemy. This is backwards compatible with earlier versions of SQLAlchemy, so doesn't explicitly require 1.3, it just won't handle unicode values correctly on <1.3. Does not impact other db engines, aka minimal risk of regressions. Fixes #6624

Before:

After:

[villebro]

Ready for review @mistercrunch

[codecov-io]

Codecov Report

Merging #6690 into master will decrease coverage by 17.34%.
The diff coverage is 70%.

@@             Coverage Diff             @@
##           master    #6690       +/-   ##
===========================================
- Coverage   73.42%   56.07%   -17.35%     
===========================================
  Files          75      525      +450     
  Lines       10084    23229    +13145     
  Branches        0     2777     +2777     
===========================================
+ Hits         7404    13026     +5622     
- Misses       2680     9794     +7114     
- Partials        0      409      +409

Impacted Files	Coverage Δ
superset/connectors/sqla/models.py	78.17% <100%> (+0.08%)	⬆️
superset/db_engine_specs.py	54.11% <57.14%> (+0.02%)	⬆️
superset/assets/src/components/Checkbox.jsx	100% <0%> (ø)
...ations/deckgl/layers/Polygon/PolygonChartPlugin.js	0% <0%> (ø)
...ets/src/dashboard/components/dnd/DragDroppable.jsx	97.14% <0%> (ø)
...c/visualizations/deckgl/layers/Polygon/Polygon.jsx	0% <0%> (ø)
...ssets/src/visualizations/presets/MapChartPreset.js	0% <0%> (ø)
superset/assets/src/components/EditableTitle.jsx	77.14% <0%> (ø)
...assets/src/visualizations/Iframe/transformProps.js	0% <0%> (ø)
superset/assets/src/setup/setupPlugins.js	0% <0%> (ø)
... and 442 more

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[villebro]

Bad rebase, back to WIP

[villebro]

This should be ok now, unsure why CI failed the cypress tests. @kristw: The .SQLLab tag should be removed, as this isn't related to SQL Lab, only query generation on MS SQL.

[xxcmp1977]

I just found that it's ignoring unicode string prefix N when using where_in clause in Jinja template
so that
and path in {{filter_values('Category')|where_in}}
expanded to
and path in ('á é í ó ú ü ñ')
instead of
and path in (N'á é í ó ú ü ñ')

Workaround is to manually rewrite it to something like

{% set values = filter_values('Category') %}
and path in (
    {% for value in values %}
        {{ "N'" + value + "'" }}{% if not loop.last %}, {% endif %}
    {% endfor %}
)

I'm using mssql+pymssql connection
Superset version 2 (not sure whuch one exactly)