Refactor Transitfeeds logic & source

[lauriemerrell]

Spinout from two comments on #78 -- we should:

• Stop reading from transitfeeds.com directly from the transitfeeds_schedule_zipfiles_raw/ directory in the s3 bucket (comment)
• Consider cutting over from transitfeeds to our downloaded data as of September 2023 instead of December (comment)

[lauriemerrell]

If we do this, maybe we can delete beautifulsoup dependency? 🤯 🙌 per @haileyplusplus