Standardize "active" constant

[chao2zhang]

While working on #3274, I noticed that I cannot find the scope where I need to add severity as a universal config key.
So I created this PR to use Config.ACTIVE_KEY as the root. Please keep in mind that there are still "active" as the string literals not touched in this PR. Those appearances are primarily test code and website generation code.

[codecov]

Codecov Report

Merging #3292 (d52c741) into master (2e89bd4) will not change coverage.
The diff coverage is 57.14%.

@@            Coverage Diff            @@
##             master    #3292   +/-   ##
=========================================
  Coverage     80.02%   80.02%           
  Complexity     2667     2667           
=========================================
  Files           443      443           
  Lines          8126     8126           
  Branches       1542     1542           
=========================================
  Hits           6503     6503           
  Misses          796      796           
  Partials        827      827           

Impacted Files	Coverage Δ	Complexity Δ
...tlab/arturbosch/detekt/api/internal/EmptyConfig.kt	80.00% <0.00%> (ø)	4.00 <0.00> (ø)
...ab/arturbosch/detekt/core/config/FailFastConfig.kt	36.36% <0.00%> (ø)	5.00 <0.00> (ø)
...lin/io/gitlab/arturbosch/detekt/api/ConfigAware.kt	66.66% <100.00%> (ø)	0.00 <0.00> (ø)
...lab/arturbosch/detekt/core/FileProcessorLocator.kt	69.23% <100.00%> (ø)	5.00 <0.00> (ø)
.../arturbosch/detekt/core/reporting/ReportLocator.kt	92.30% <100.00%> (ø)	4.00 <0.00> (ø)
...io/gitlab/arturbosch/detekt/core/rules/RuleSets.kt	100.00% <100.00%> (ø)	0.00 <0.00> (ø)

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[schalkms]

Thanks for the refactoring. Looks good to me. 🙂