Add individual overground lines

[martincostello]

• Add colours for the new individual overground lines.
• Use CSS variables for the line colours.
• If someone still has london-overground as a preference, automatically return all the new individual lines instead.
• Fix DLR appearing before District.

[costellobot]

Device	URL
desktop	https://localhost:50001

Device	URL
mobile	https://localhost:50001

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Codecov Report

Attention: Patch coverage is 26.66667% with 11 lines in your changes missing coverage. Please review.

Project coverage is 77.46%. Comparing base (c8228ce) to head (95df183).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/LondonTravel.Site/ApiModule.cs	15.38%	10 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2932      +/-   ##
==========================================
- Coverage   77.86%   77.46%   -0.40%     
==========================================
  Files         123      123              
  Lines        2503     2512       +9     
  Branches      380      381       +1     
==========================================
- Hits         1949     1946       -3     
- Misses        371      381      +10     
- Partials      183      185       +2     

Flag	Coverage Δ
linux	77.46% <26.66%> (-0.40%)	⬇️
macos	77.46% <26.66%> (-0.40%)	⬇️
windows	77.42% <26.66%> (-0.40%)	⬇️

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