Remember last selected indicator.

Fixes TODO by using session cookie to persist last indicator selection through
page refresh.
Refactor indicators into separate classes
Increase spray timeout to 20 minutes
Move durations helper to TimeTraveledStops class
Merge pull request #107 from WorldBank-Transport/feature/refactor-indicators

Refactor indicators into separate classes
Merge pull request #108 from flibbertigibbet/feature/cleanup-windshaft-indicators

Remember last selected indicator.