MAILBOX-392 Relax mailbox name constraint regarding '&'

As per RFC-2060 '&' character can be escaped

```
   In modified UTF-7, printable US-ASCII characters except for "&"
   represent themselves; that is, characters with octet values 0x20-0x25
   and 0x27-0x7e.  The character "&" (0x26) is represented by the two-
   octet sequence "&-".
```

As such, the '&' don't need to be forbidden in mailbox name.

This caused issue during some data migration as the use of & in folder names is popular.
6 files changed