feat: add vendor info and filter to reports site

[ryon]

Description

• Modify generate.py to aggregate vendor names from feed_info files and pass them into other templates (reviewers: is this the best approach?)
• Add a section listing vendors in individual reports
• Add "vendor" filter to monthly index pages
• Refactor filter system so it's easier for developers to read and extend, and for users to interact with

Resolves #181

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation

How has this been tested?

I tested this functionality locally, using data synced down from Google Cloud.

Screenshots (optional)

New vendor filter and info bar that shows when a filter is active

Vendor info section on individual reports

Vendors that are listed in both "schedule" and "realtime" sections will also be grouped appropriately

[github-actions[bot]]

Preview url: https://PR-250--cal-itp-reports.netlify.app

[ryon]

@edasmalchi Yep, it's the small data set. That yellowish info bar will only show up when at least one agency has been filtered out, so in this case you can still trigger it using the search bar.