Store-wide reset on login/logout

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The most important part is in js/store/index.js: The whole store module handling is rewritten to be dynamic.

When a new store module is added, a string is added to the moduleNames array. A modules object is then built, dynamically importing everything. The main use of this array is that we can loop through it and call mutations on each module; that's how the store-wide reset works.

All states in modules are replaced by initialState functions, to create new objects and always trigger Vue's reactivity.

All the other changes are about renaming clear mutations to reset, updating tests, and making the auth module trigger resets on login and logout.