Source Editor: The toolbar Vue components

This is the second leg of the toolbar MVC for the Source Editor:

• Source Editor toolbar feature flag
• Source Editor: The toolbar Vue components: this MR
• Source Editor: The toolbar integration (future MR)

This MR introduces:

• The two new Vue components:
  • The toolbar button
  • The toolbar itself
• GraphQL Apollo support for the local state management

Note: this app is purely client-side driven so we rely on the Apollo cache for the state management and don't query for any other GraphQL data.

Why not Vuex?

We would like to use the final technologies right away in every state of the integration. Otherwise, there's no technical limitation to using one solution over another.

This MR does not have integration yet: it will be introduced in the next step. But anyway, this whole work will be hidden behind the feature flag, hence the absence of the changelog entry