Admin Users: persist requested storage, rename and regroup filters, add clear action

[akolson]

Summary

This fixes QA follow-ups from #5946, all on the admin Users page.

• Requested storage never reached the "Storage needed" column. information["space_needed"] is only written at registration, and only when the user ticks "Storing materials for private or local use" — the post-registration storage request flow just sent an email and persisted nothing. So users who requested storage after signing up showed blank, while older users who answered the registration question showed a value. StorageSettingsView now records the requested amount on the user, and the CSV prefers it over the registration answer. No migration: information is an existing JSONField.
• "Has Studio edits" was misnamed. The filter is Exists(Change.objects.filter(created_by=...)), which matches any Change row the user ever created — creating a channel qualifies, and deleting it later doesn't undo it. That is activity, not edits, which is what QA observed. Renamed in the filter row and the CSV header; the query param and backend annotation are unchanged, so existing bookmarked URLs still work.
• The two checkboxes sat in separate quarter-width columns, leaving the wide gap in QA's screenshot. They now share one half-width flex row with a "Clear filters" action at the end.
• Clear filters removes every filter while preserving pagination and sorting. It is enabled only once a filter differs from the default its control already displays — so choosing "All" for user type enables it (the select is blank until you pick something), while "Any time" on the date windows, or ticking and unticking a checkbox, does not.

References

Fixes #5946

Reviewer guidance

Manually verified against a local devserver at 1600px and 820px, signed in as an admin: the renamed label, the two checkboxes now adjacent, "Clear filters" greyed out on a clean page, enabled after selecting "All", and the row wrapping cleanly at the narrow width. Screenshots to follow in a comment.

AI usage

Used Claude Code throughout, in a plan-then-review loop. I had it trace the root cause of the empty "Storage needed" column before proposing anything, and rejected its first design, which stored a requested_on timestamp nothing would read. I also caught that its clear-filters logic left the action disabled after selecting "All"; the fix was to compare each filter against the default its control already shows rather than against the params sent to the backend. Its first attempt at that comparison marked a clean page as filtered — the existing test suite caught it. I reviewed every diff, ran the frontend and backend suites and the linter, and verified the layout in a browser.