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/ja.ts

@@ -998,6 +998,8 @@ export const catalog: Catalog = {
     "folder": "フォルダー",
     "“{name}” moved into “{parent}”": "「{name}」を「{parent}」に移動しました",
     "“{name}” moved to the top level": "「{name}」を最上位に移動しました",
+    "Move “{name}” to…": "「{name}」の移動先…",
+    "Top level": "最上位",
     "Could not move “{name}”: {reason}": "「{name}」を移動できませんでした: {reason}",
     "Delete “{name}”?": "「{name}」を削除しますか？",
     "Rename folder": "フォルダー名を変更",