Reduce number of available licenses

[rob006]

Right now license sector is overwhelming and contains too many options (in random order), and many of them are just bad options for open source extension. This increases chances that someone unfamiliar with licensing will pick problematic license. I suggest to limit options to the most popular ones, and set MIT as default. Some overview of the most popular licenses can be seen at https://rob006-software.github.io/flarum-translations/status/licenses.html