quarantine: refactor spamquarantine controller

quarantine: refactor spamquarantine controller

over the time the spam quarantine has gained quite a few nice
usability improvments and features, which would be useful in
the virus and attachment quarantines as well - e.g.:

* multi-mail selection
* keyboard actions
* context menu
* download mail as .eml

this patch splits the controller part into a file of its own, while changing
'var' to 'let' and removing the parts only relevant for the spamquarantine
and adapts the current SpamQuarantine.js to use it.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>