refs https://github.com/TryGhost/Team/issues/1277
- renamed `<GhActivityTimeline>` to `<GhMemberActivityEventParser>` and modified so that it yields parsed events rather than directly renders them
- makes the component re-usable as it can be used to decorate raw events ready for use in context-specific templates
- switches to using a getter to yield the parsed events so that they will update automatically when the `@events` argument changes
- updated `<Dashboard::LatestMemberActivity>` to use `<GhMemberActivityEventParser>` and keep the member activity box output local to itself
- added integration tests for `<Dashboard::LatestMemberActivity>`
- added Mirage setup for member activity event models/serializers/route
refs https://github.com/TryGhost/Team/issues/1277
- first step of further refactoring to make member activity display more generic
- separates component data loading from overall controller logic and allows it to be tested in integration tests as well as acceptance tests
