Remove feature flag checks for vitalsource.page_ranges flag

[robertknight]

This enables VitalSource page range selection for all installs. The consequences of this are:

• When configuring a VitalSource assignment, there is the option to "Choose a chapter or page range" instead of "Pick a place to start reading"
• In the assignment, we show only annotations from the selected chapter or page range, and show a toggle button in the client to choose whether to show annotations from the rest of the book
• When users navigate outside the selected chapter / page range, they are shown a notification that any annotations they create there may not count towards the assignment, but they are still able to annotate.
• Existing VitalSource assignments are unaffected, and work the same as before

[robertknight]

I'm going to hold off merging this until others have had a chance to test a fix for a problem reported in https://hypothes-is.slack.com/archives/C4K6M7P5E/p1710190672323349.

[robertknight]

Merging as discussed in https://hypothes-is.slack.com/archives/C2C2U40LW/p1710766017270399.