Increase diff components reusability

Right now Diffs app is tightly coupled with the Notes app which handles Issues. That creates a problem when we want to re-use the Diffs components anywhere else.

That is caused by using:

1. Notes app components that rely on a Vuex state
2. Mixins that also rely on a Vuex state
3. Heavily relying on a Vuex state in our own Diffs components

In order to tackle this and increase reusability and maintainability we need to do a major refactoring of the code:

1. Replace mixins with composables (should be possible after Vue 3 migration)
2. Replace Vuex with composables where possible
3. Replace Vuex with props\events if necessary

Refactoring details to be defined.