feat: hide CR timetable in the presence of special alerts

[paulswartz]

Summary of changes

Asana Ticket: CR disruption timetable improvements

This adds special-case handling for two types of alerts where we want to hide the timetable from view. One case is expected to have a PDF as the URL for the alert, and the other does not. In the PDF case, we hide the timetable and provide the link to the PDF. In the other, we hide the timetable and say "PDF upcoming."

This also changes the rendering of the alert to drop the special-case text, and to say "View PDF schedule" if the PDF link is available.

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General checks

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New UI, or substantial UI changes

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New endpoints, or non-trivial changes to current endpoints

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• If this change involves routes, does it work correctly with pertinent "unusual" routes such as the combined Green Line, Silver Line, Foxboro commuter rail, and single-direction bus routes like the 170?

[paulswartz]

@thecristen I made the changes as fixup! commits; I'll clean up the history after approval.

[thecristen]

Thanks!!

[thecristen]

Love it!! Thanks so much 👍🏼