Styling changes + menu/modal unification + code cleanup

What started out as an effort to style up the modals turned into a quest to unify the way the various menus and modals are presented and styled. Only a minimal amount of styling was actually done, but all of the modals are now derived from the same set of CSS rules, so we can style them up in the future with ease while adhering a common theme.

Also added was a function to center the modals more accurately when the page is really tall. The main menu, pause and scenario select modals all display in the same place and with similar styling.

Finally, minimap.js and resizer.js were cleaned up.