Rename Members List to Memberships and add Orders-style filter panel

[dparker1005]

What this does

Terminology: Members List → Memberships

Now that a single user can hold multiple levels, each row in this list is a membership (user + level), not a member — the query has grouped by u.ID, mu.membership_id for a while, so the labels are catching up to the code.

• Admin submenu, admin bar node, header nav tab, and page H1 renamed to Memberships.
• List table singular/plural, "Memberships per page" screen option, and "No memberships found." empty state updated.
• Person-oriented language is intentionally kept: Add New Member, Search Members, Edit Member.
• No changes to slugs (pmpro-memberslist), capabilities, or hook names — existing links and add-ons are unaffected.

Filtering: same filter panel as Orders and Subscriptions

Replaces the single overloaded dropdown (which mixed levels with Cancelled/Expired/Old Members and could not combine them) with the pmpro-filter-panel sidebar used on the Orders and Subscriptions pages:

• Level — any level, now combinable with status (e.g. "expired Gold memberships").
• Status — Active (default) / Cancelled / Expired / All Ended. "All Ended" uses an explicit status list from the new filterable pmpro_get_ended_membership_statuses() (expired, cancelled, admin_cancelled, changed, admin_changed, inactive) rather than <> 'active', so incomplete-checkout statuses (token, pending, review, error) can't leak in.
• Ended Memberships (shown only for ended statuses) — "Hide if the same level is now active" (default) or "Hide if any level is now active".

Ended-view semantics

Each row represents the current state of a user's relationship with a level, so ended views always hide a row when that same level has been re-activated (a user who expired out of Gold and re-purchased Gold is not an "expired membership"). The old behavior — only show users with no active membership at all — is the "any level" option. This fixes the long-standing quirk where a user who downgraded from Gold to Silver never appeared anywhere as an ended Gold membership.

Shared query semantics for list + export

New pmpro_sanitize_memberships_list_filters() and pmpro_memberships_list_filter_sql() helpers are used by both the list table and the members CSV export, so exported rows always match the rows on screen.

Backwards compatibility

• Legacy l=cancelled, l=expired, and l=oldmembers URLs (dashboard, reports, admin activity email, add-ons) are mapped to the new status/excludeactive params with their original no-active-membership behavior preserved.
• Numeric l values behave exactly as before (level + active).
• pmpro_members_list_sql and all column/CSV hooks fire unchanged.

Bug fix along the way

column_enddate() keyed off the legacy l values, so ended views reached via the new status param rendered blank end dates; it now uses the normalized status filter.

Testing

1. Go to Memberships > Memberships. Confirm the default view shows active memberships and the Filter Results button opens the sidebar panel.
2. Create a user with an expired level A and an active level B. Filter Status = Expired: the expired A row shows with "Hide if the same level is now active" and hides with "Hide if any level is now active".
3. Create a user with an expired level A who re-purchased level A. The expired row is hidden under every ended view.
4. Visit a legacy URL like admin.php?page=pmpro-memberslist&l=expired and confirm it matches the pre-change results (only users with no active membership), with the panel reflecting Expired + "any level".
5. Combine a Level with Status = Expired/Cancelled/All Ended and confirm results, sorting by end date, and the End Date column values.
6. Export to CSV from a filtered view and confirm the rows match the screen.

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