Allow specifying custom width for logo

[betodealmeida]

We currently allow user to specify a custom logo for Superset, but the width is fixed at 126 pixels. I added an option to specify a different width in config.py.

[betodealmeida]

cc: @xtinec

[codecov-io]

Codecov Report

Merging #6739 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #6739      +/-   ##
=========================================
+ Coverage    56.1%   56.1%   +<.01%     
=========================================
  Files         521     521              
  Lines       23195   23196       +1     
  Branches     2777    2777              
=========================================
+ Hits        13014   13015       +1     
  Misses       9772    9772              
  Partials      409     409

Impacted Files	Coverage Δ
superset/config.py	93.5% <100%> (+0.04%)	⬆️

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[codecov-io]

Codecov Report

Merging #6739 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #6739      +/-   ##
=========================================
+ Coverage    56.1%   56.1%   +<.01%     
=========================================
  Files         521     521              
  Lines       23195   23196       +1     
  Branches     2777    2777              
=========================================
+ Hits        13014   13015       +1     
  Misses       9772    9772              
  Partials      409     409

Impacted Files	Coverage Δ
superset/config.py	93.5% <100%> (+0.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1fece0d...2b323e8. Read the comment docs.

[kristw]

@betodealmeida I updated CONTRIBUTING.md with how to use labels. Could you take a quick look? #6701

[betodealmeida]

@kristw taking a look. Sorry, I missed that discussion.