Use existing icon for some old/unused icons

• i/search, copy of i/preview.
• i/files, copy of i/folder.
• i/calendar, copy of i/siteevent.
• i/db, copy of i/repository.
• t/user, copy of t/groupn.

Not used in the core, but the name is generic enough not to be deprecated, and we have an icon that would suit for them, so why not using them.