Added launch date to overallStats API call

[davphan]

Resolves #3375

Launch date of a city is currently not available. To show this information, the date of the first label in each city was found and added to the city params config file. The info is then drawn from the config file to the API call.

Before/After screenshots (if applicable)

Before:
No info displayed for city launch date.

After:
CSV format:

JSON format:

Testing instructions

1. Use the /v2/overallStats and /v2/overallStats?filetype=csv API calls.
2. Check that the launch_date/Launch Date fields are displayed correctly.

Things to check before submitting the PR

• I've written a descriptive PR title.
• I've added/updated comments for large or confusing blocks of code.
• I've included before/after screenshots above.

[misaugstad]

Thanks, just a quick restructuring thing to do!

[davphan]

@misaugstad Ready for another review!

[misaugstad]

Looks good, thanks @davphan!

I did make two minor changes that you should look out for in the future.

1. There was an import statement left over from before the recent refactor. You can make sure stuff like that doesn't happen by looking over the "files changed" tab of your PR after you finish making your edits. It's a good idea to read through all those changes to make sure there's no extra fluff that you left in!
2. I added type annotations to a couple variables. This is strongly encouraged in Scala. I'd generally like for you to use them almost everywhere. One exception is for Slick query objects; those can have really long, uninformative data types. The other exception is for edge cases where it would significantly decrease the readability of the code, while the data type is obvious from context. What I changed were the two Play.configuration.getString lines to make them look like val cityId: String = ..., etc.