Small CSS fix

The new view HTML/plaintext icons inside #messageheader when viewing a HTML message with Roundcube 1.0.1 mean that we need a wider CSS margin on the message subject.

Otherwise a long subject line runs behind the two new icons and the "..." ellipsis at the end is lost.

The problem shows up frequently with the threecol plugin installed:

    https://github.com/tofi86/Roundcube-Plugin-Threecol-Layout

(I have submitted a pull request to make that work with 1.0.1)

However you see the same problem with the standard mailbox view if a message has a very long subject line.