Make it possible to view messages of the same thread directly

css/mail.scss

css/mobile.scss

src/components/AddressList.vue

src/components/MailboxThread.vue

src/components/Message.vue

src/components/Thread.vue

src/components/ThreadEnvelope.vue

src/store/actions.js

src/store/getters.js

src/store/mutations.js

src/tests/unit/store/getters.spec.js

src/tests/unit/store/mutations.spec.js