Material UI: upgrade modals

Also clean up the Create Torrent page, delete some redundant CSS, prevent click-and-drag inside a TextField from moving the whole window, and make all label and input fonts  a consistent 14px size.

src/renderer/components/path-selector.js

@@ -62,8 +62,7 @@ class PathSelector extends React.Component {
       color: colors.grey50
     }
     const textFieldStyle = {
-      flex: '1',
-      fontSize: 14
+      flex: '1'
     }
     const text = this.props.displayValue || this.props.value
     const buttonStyle = {