Highlight clicked button

[Waldir Pimenta]

This would remove any doubts regarding what button was clicked, which is especially relevant in mobile, since our fingers are rather imprecise pointers and indeed more than once, while playing the game in my mobile phone, I was not sure which button I had clicked.

Maybe Bootstrap already does this, but I didn't notice it.

[Magnus Manske]

Making button font bold, and deactivating the other buttons

[Waldir Pimenta]

For future reference, this was done in fd0d1e6.