Add highlight toggle for block/text highlight

[ejunga]

Previously, the student and main userscripts had incompatible highlighting methods, e.g. for !! and ~~ highlighting. The main userscript would highlight the whole block, preserving only the text within the delimiters, while the student userscript would highlight only the text, but preserve all of the text within the block.

These edits add a toggle within the userscript settings menu to switch between these two highlighting methods similar to how the link behavior is handled. It defaults to the previous block highlighting scheme, but can be switched to the text behavior with a simple toggle of the radio buttons, saving that config to local storage with the rest of the configuration options.