Store reusable types and interfaces in a central directory

[eddybrando]

We currently have some type and interface definitions stored in very specific places (like the store directory), which could and should be reused in other parts of the codebase outside from those.

An example for this is the Advocate type, which lives in the Vuex store directory but is also imported and used outside of the Vuex context, by Vue Components for instance.

As discussed in #1122 (comment), we should refactor all reusable types and interfaces in the codebase and store them somewhere more central.

Additionally, as pointed out in #1122 (comment), we should discuss whether the most appropriate place for this would be indeed the types directory in the root or if there is a better alternative. For this, we should probably start by researching Nuxt and Vue.js TypeScript directory structure best practices.

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Related to #1122