[MBL-993] Dark Mode Feature Flag

[scottkicks]

📲 What

A new Feature Flag for the Dark Mode

🤔 Why

Having a safeguard for implementing dark mode allows us to more confidently release and rollout this feature.

🛠 How

• Updated RemoteConfigFeatures, RemoteConfigFeature+Helpers, RemoteConfigFeatureFlagToolsViewModel.
• Added tests

dark_mode was also added to our Firebase RemoteConfig Dashboard Production, Beta and Alpha apps.

👀 See

Trello, screenshots, external resources?

Before 🐛	After 🦋

✅ Acceptance criteria

• Feature flag is available for use via the featureDarkModeEnabled() helper method.
• Feature flag is available in our Firebase RemoteConfig Dashboard
• Updating the flag from our Firebase dashboard updates the app
• We can update the app locally. This change persists and takes priority over the firebase dashboard flag value. This is for internal testing.

[codecov]

Codecov Report

Merging #1865 (8692c86) into main (1475571) will decrease coverage by 3.96%.
The diff coverage is 74.07%.

@@            Coverage Diff             @@
##             main    #1865      +/-   ##
==========================================
- Coverage   87.91%   83.95%   -3.96%     
==========================================
  Files         858     1222     +364     
  Lines       76777   111055   +34278     
  Branches    20310    29506    +9196     
==========================================
+ Hits        67501    93241   +25740     
- Misses       8538    16801    +8263     
- Partials      738     1013     +275     

Files	Coverage Δ
...oteConfigFeatureFlagToolsViewControllerTests.swift	100.00% <100.00%> (ø)
...emoteConfig/RemoteConfigFeature+HelpersTests.swift	100.00% <100.00%> (ø)
...s/RemoteConfigFeatureFlagToolsViewModelTests.swift	100.00% <100.00%> (ø)
...ary/RemoteConfig/RemoteConfigFeature+Helpers.swift	80.00% <80.00%> (ø)
Library/RemoteConfig/RemoteConfigFeature.swift	0.00% <0.00%> (ø)
...Models/RemoteConfigFeatureFlagToolsViewModel.swift	69.13% <37.50%> (-3.47%)	⬇️

... and 368 files with indirect coverage changes

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[amy-at-kickstarter]

LGTM