Migrate to Composition API

[annybs]

Ref. previous discussion

Vue's new Composition API is generally easier to read and write than Options API, as it speaks JS rather than Vue's internal schematics. This is the general direction of Vue maintainers' own efforts and we should follow suit.

Newer components being added to this project are already using Composition API. Some existing components have been migrated recently in the course of work on billing. Other components should follow suit over time when appropriate occasions arise - especially when carrying out #261 migrations.