Move a folder from its menu, with the same picker as moving mail

A folder could only be moved by dragging it, which is slow in a long list
and not offered at all on a touch screen. Its menu now has "Move to…",
which opens the searchable folder picker that moving messages already
uses, with a "Top level" row above the folders.

The picker lists only legal destinations: the same rules as a drop -- not
into itself, its own subtree or the parent it already has -- plus the
rights a picker has to check up front because it shows every folder at
once: mayRename on the folder being moved, which RFC 8621 uses for
reparenting, and mayCreateChild on the destination. Both only say no on
shared mail. The move itself goes through the same path as a drop, so the
toast and the expanded destination are unchanged.

"Move “{name}” to…" and "Top level" are in all nine catalogues.

Closes #355

web/src/locales/ru.ts

@@ -939,6 +939,8 @@ export const catalog: Catalog = {
     "folder": "папка",
     "“{name}” moved into “{parent}”": "«{name}» перемещена в «{parent}»",
     "“{name}” moved to the top level": "«{name}» перемещена на верхний уровень",
+    "Move “{name}” to…": "Переместить «{name}» в…",
+    "Top level": "Верхний уровень",
     "Could not move “{name}”: {reason}": "Не удалось переместить «{name}»: {reason}",
     "Delete “{name}”?": "Удалить «{name}»?",
     "Rename folder": "Переименовать папку",