Modularise MULLE

[heatherleaf]

Better separation between frontend, database backend and MUSTE grammar backend.
We need to make both the frontend and the backend more modular.

The frontend is currently located inside muste-ajax/static, but should be completely separated from the Haskell code. Also, the frontend should be easier to change.

• the Ajax backend should separate the Configuration from the Database from the Similar-trees from the Menu-creation

• move muste-ajax/static to top-level

• move the lesson-specific i18n files to where the config files are

• the frontend should make the exercise view modular, so that it's easy to change exercise types

The API to the database and MUSTE should be separated. It should be possible to have the DB and MUSTE on separate servers, and the DB does not have to be in Haskell.

• move the DB code to a new Stack project muste-db

• (perhaps) change the DB source code to Python or JS instead of Haskell

The static html/js files should not have to be served by the Haskell web server - we want to be able to have them and the api on different servers

• make it possible to use cross-origin requests, and to put the static files on any web server

[heatherleaf]

the backend is modularized in 731e343