minor changes, standardizing mixin naming
Fix for bug introduced in f9d6fb1 (2nd to last commit)
`app.jsx`: minor change to support new `chooseScreen` mixin

`gatherings.jsx`: Moved lots of code into a new `chooseScreen` mixin (to reduce code duplication between `gatherings` & `heroes`)

`heroes.jsx`: Ditto

`app.scss`: Minor change

`src/script/mixin/chooseScreen.js`: new mixin