Hide filter toggles irrelevant to the selected city

[govindlahoti]

This commit -

1. Hides the layers that are irrelevant for the selected city. Previously all layers for all cities were being shown.
2. Hides the transit lines that are irrelevant for the selected city. Previously all transit lines for all cities were being shown.
3. Introduces Cities enum for type-safe way of specifying city anywhere in the code. Previously strings were being used which can easily lead to errors.
4. Replaces the use of string with Cities enum for Available Routes and Available Regions records.

This commit fixes the issue mentioned at #40

Here are the screenshots showing the desired UI change -