Add tera raids specific functionalities/battle format

[davbou]

Hi! Your calculator has proven to be an invaluable tool for communities of tera raiders like the ones I'm in currently.

Unfortunately, some tera raids specificities and functionalities are missing, such as cheers, stacking steely spirits, boss health multipliers, etc.

I implemented a lot of those in a fork I maintain for a community of tera raiders I'm in which you can test in action here (repo link) however, we also made some other changes(such as hiding the competitive sets, and minor changes to the css for mobile users) to better suit our needs that would need to be cleaned up before submitting a proper PR.

So, before cleaning it up, is a tera raid battle format with those added functionalities something you would even want to incorporate in the main calculator or should I keep it in a separate fork as I'm doing right now?