Introduce feature flags

[Process-ing]

Closes #872
I included some example flags to clarify the normal use-case, they should be removed later when real feature flags are to be included

Workflow

1. Add the feature flag info inside featureFlagTable
2. Retrieve the feature flag to the UI component, using featureFlagController.getFeatureFlag(<code>) and save it in a static field (if you receive the error Null check operator used on a null value, it probably means the feature flag code is mismatched)
3. Wrap the experimental components inside ExperimentalFeatureWrapper, and associate the latter to the feature flag
4. Toggle the feature flags in the app settings to see the results

Review checklist

• Terms and conditions reflect the current change
• Contains enough appropriate tests
• If aimed at production, writes a new summary in whatsnew/whatsnew-pt-PT
• Properly adds an entry in changelog.md with the change
• If PR includes UI updates/additions, its description has screenshots
• Behavior is as expected
• Clean, well-structured code

[codecov]

Codecov Report

Attention: Patch coverage is 1.88679% with 156 lines in your changes missing coverage. Please review.

Project coverage is 11%. Comparing base (9acce2d) to head (2c52f36).
Report is 5 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #1092    +/-   ##
========================================
- Coverage       11%     11%    -0%     
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  Files          269     278     +9     
  Lines         7345    7494   +149     
========================================
+ Hits           802     805     +3     
- Misses        6543    6689   +146     

[LuisDuarte1]

Could you add "group" feature flags? For instance, the feature flag new-redesign will activate/deactivate many feature flags like: courses-page, and schedule-page. To make feature flags easier to interact with.

Resolved