Display order transaction IDs and allow search by ID in the WordPress admin

[dwanjuki]

Migrating snippet to library: https://gist.github.com/strangerstudios/898055f2d6f5a9676bf1

Resolved PHP warnings that would occur when searching by Transaction ID on members list page:
Undefined array key "display_name"
Undefined array key "s"

Removed code to add Subscription Transaction IDs to Members List CSV export. PMPro core already includes this column.

Updated to use newer pmpro_manage_memberslist_columns and pmpro_manage_memberslist_custom_column instead of pmpro_memberslist_extra_cols_header and pmpro_memberslist_extra_cols_body hooks.

Now compatible with multiple memberships per user. With original snippet, if a member has multiple memberships, the member's most recent transaction ID would display in all of their membership rows in the Members List and Export CSV.

Added some escaping and WPCS.