stop using vue.set

src/components/font_control/font_control.js

src/components/gallery/gallery.js

src/components/settings_modal/tabs/theme_tab/theme_tab.js

src/modules/chats.js

src/modules/config.js

src/modules/instance.js

src/modules/interface.js

src/modules/oauth.js

src/modules/polls.js

src/modules/statuses.js

src/modules/users.js