[IMPROVEMENT] Add toggle markdown to settings (#907)

app/actions/actionsTypes.js

app/actions/markdown.js

app/containers/message/Markdown.js

app/i18n/locales/de.js

app/i18n/locales/en.js

app/i18n/locales/fr.js

app/i18n/locales/pt-BR.js

app/i18n/locales/pt-PT.js

app/i18n/locales/ru.js

app/i18n/locales/zh-CN.js

app/lib/rocketchat.js

app/reducers/index.js

app/reducers/markdown.js

app/sagas/init.js

app/views/RoomView/index.js

app/views/RoomsListView/index.js

app/views/SettingsView/index.js