Support "Mark as important" in filters

[adambkaplan]

When exporting filters, it appears that IMPORTANT is treated as one of the special label IDs, alongside INBOX, UNREAD, and TRASH.

The gmail filter UI appears to support IMPORTANT in two ways:

1. Always mark the email as important (add label)
2. Never mark the email as important (remove label)

We should therefore support the following:

1. AlwaysMarkImportant
2. NeverMarkImportant