Force quotes on non-expression time grains on Postgres

[villebro]

Due to time grains being rendered as literal columns in sqla/models.py, engine-specific quoting logic is not applied. This causes mixed-case columns to fail on Postgres in timeseries graphs if a time grain is applied. There have been attempts to fix this by forcing quotes in the time grain; however, this caused time grains on expressions to stop working (see #3844 for context). This PR introduces a function in db_engine_specs.py to handle rendering of the expression being passed to the time grain, which for Postgres forces quotes around MixedCase column names, but not lower_case. This has been tested to work in all scenarios:

• expression with time grain
• expression without time grain
• column with time grain
• column without time grain

Fixes #5886

Before:

After:

[codecov-io]

Codecov Report

Merging #6897 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6897      +/-   ##
==========================================
+ Coverage   63.96%   63.97%   +0.01%     
==========================================
  Files         422      422              
  Lines       20524    20533       +9     
  Branches     2230     2230              
==========================================
+ Hits        13128    13137       +9     
  Misses       7264     7264              
  Partials      132      132

Impacted Files	Coverage Δ
superset/db_engine_specs.py	56.75% <100%> (+0.44%)	⬆️
superset/connectors/sqla/models.py	81.73% <100%> (+0.03%)	⬆️

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[mistercrunch]

Unit tests please!

[villebro]

Added test cases for expression, lower_case and MixedCase column names that mimic what's going on in get_timestamp_expression()in sqla/models.py.

[rumbin]

@villebro, thanks a lot for sorting this quoting issue out comprehensively

[villebro]

FYI @mistercrunch rebased to resolve conflicts