Add virtual priority inbox

lib/Contracts/IMailSearch.php

lib/Controller/MessagesController.php

lib/Db/MessageMapper.php

lib/Service/Search/MailSearch.php

lib/Service/Sync/SyncService.php

src/components/Mailbox.vue

src/components/MailboxMessage.vue

src/components/NavigationFolder.vue

src/components/SectionTitle.vue

src/i18n/MailboxTranslator.js

src/service/MessageService.js

src/store/actions.js

src/store/constants.js

src/store/index.js

src/store/mutations.js

tests/Unit/Service/Search/MailSearchTest.php