Fixed Safari .webarchive backdrop problem

Reopening a saved Safari webarchive of a mail content already decrypted
with Xeit had a div.modal-backdrop which obscures the entire page.  This
fixes the issue by showing the authentication dialog only if there were
no mail-content yet.
Merge branch 'webarchive-support' of https://github.com/netj/xeit into webarchive (#55)
Use another approach (possibly with less side-effect) for webarchive detection
Refactor `isMailAttached()`
Merge pull request #58 from tomyun/webarchive

Safari webarchive 지원