Improve "Use Bridges" UI based on feedback and testing

We're likely to run into a few issues with the "Use Bridges Bridges" UI once users in various localizations try to use it. One issue we've already noticed is that people can become confused by the type selection dropdown, and may think it applies to bridges they enter in the "Enter custom bridges" textbox. We should probably make these radiobuttons behave such that they are more clearly mutually exclusive (ie when one is selected, all children of the other are greyed out).

I've tried to improve the situation a little with basic layout changes, but I am not sure those won't look worse in RTL languages and in very verbose localizaions, so that may be another issue to address: https://people.torproject.org/~mikeperry/images/Settings.jpg