Fix inconsistent add torrent dialog window width

[drrako]

Overview

This PR fixes a minor issue in the add torrent dialog, where its size changes depending on the selected input torrent file (especially in Firefox). Some long torrent file names with many space breaks can significantly stretch the modal window.

I've also updated styles so this window always take 95% of the viewport on a smaller screens.

[allixx]

JFYI, pull request should probably be based on develop branch. On 2nd thought, I am really not sure if that's a bugfix or a feature, so I'll just shut up :)

[drrako]

@allixx I considered it a simple enough bug fix for the master branch.