Dropdown selection added for IMAP & SMTP security config

reduce message list width in favor of message detail view

reload account data after adding another account

fixes #469

(attempt No. X)
As discussed by the main developers of the master branch, two dropdown
boxes have been added with subsequent prefilling of the port fields.

IMAP security "none","ssl","tls"
-> IMAP port  "143","993"

SMTP security "none","ssl","tls"
-> SMTP port "25","465"

There are new challenges by adding these boxes - it breaks the original
"design". But at least the functionality is given.

hide technical attachments like signature.asc and smime.p7s, fix #126

directly change star state in the interface for quick feedback

set 'New Message' button enabled; show loading animation

Fixing download of attachments where the filename is within the content-type and not the content-disposition 

fixing download of attachments where the filename is within the content-type and not the content-disposition - fixes #438

set 'New Message' button enabled when clicking on another message; show loading animation when switching from new message to viewing an existing one
fixes #463

NoCSRFRequired on ProxyController::proxy()

highlight only one folder at a time

Fixing escaping of sent and replied notifications

Correctly decode transfer encoding on message body

Core install link added

fix wording and capitalization

Updating authors + setting minimum dependency to ownCloud 8.0

Create sent folder automatically if it does not exist

fixes #440

ellipsize account names in sidebar/settings to fix overflow, fix #446

Fix Content Security Policy for frame-src

Fix undefined variable 'name'