[IMPROVE] Rename client-side term "Livechat" to "Omnichannel" (#16752)

app/livechat/client/route.js

app/livechat/client/ui.js

app/livechat/client/views/app/livechatAgents.html

app/livechat/client/views/sideNav/livechat.html

app/livechat/client/views/sideNav/livechatFlex.html

app/livechat/client/views/sideNav/livechatSideNavItems.js

app/livechat/server/config.js

app/ui-admin/client/rooms/adminRooms.html

packages/rocketchat-i18n/i18n/en.i18n.json

packages/rocketchat-i18n/i18n/pt-BR.i18n.json

private/public/icons.svg

public/public/icons.html

server/startup/migrations/index.js

server/startup/migrations/v176.js