Address attendee log entry requirements review feedback

[travismolick]

Follow-up to the review comments on #66.

Changes

• Keep cached registration levels when a refresh fails. Refreshing no longer forgets the cache first. The levels are fetched straight from ConCat and the cache is only replaced when that succeeds. If ConCat can't be reached, the previous list stays in use and the admin gets an error message ("Couldn't reload registration levels from ConCat."). The JSON response includes the old list plus an error field.
• Drop the redundant trim of override reasons. TrimStrings and ConvertEmptyStringsToNull are both in the global middleware stack, so blank or padded reasons are already handled. The existing "records an optional reason" test covers this (it fails if those middlewares are disabled).
• Move entry requirements into a dialog. The requirements panel is now a dialog opened by an "Entry Requirements" button in the page toolbar (admins only, hidden on read-only events), so the Attendees and Gatekeepers tables keep their full height. Saving closes the dialog; closing it without saving discards unsaved changes.

The architecture wiki page mentions the new button and the refresh behavior.

Not included

Export formula escaping (raw data for the web view vs. escaped data for exports) is left for a separate change together with the other reports.

Testing

• New test: cached registration levels are kept when a refresh fails (JSON and redirect).
• All attendee log entry requirement tests pass.
• Checked in the browser on a dev environment: opening the dialog, discarding unsaved changes on close, closing on save, reloading levels, and phone width.