Bug 800402 - [email] UI: Show the number of unread messages in the message-list header after the folder name

nextFolderNum, nextMutationNum, lastFolderSyncAt, capability, and rootDelim are all account-specific information and should not go through the makeFolderMeta code-path.  This is very regrettably horribly non-obvious because the same naming convention is used in two different data structures that happen to be crammed into the same dictionary but that vary based on the depth.

Specifically, the situation is basically:
```js
var folderInfo = {};
var accountMetaSnafu = { nextFolderNum: 0 };
folderInfo['$meta'] = accountMetaSnafu;
var someFolderId = accountMetaSnafu.nextFolderNum++;
folderInfo[someFolderId] = { $meta: makeFolderMeta() };
```

Clarifying, '$meta' is effectively a magical folder-id for the account, whereas for folders it's part of the data-structure.

I have no idea why I did this or what made it seem reasonable or sane.  My best guess is I was trying to establish a convention without realizing how tremendously confusing it would end up.