Before this commit, when clicking on a channel mention from a chat
window, it crashed with the following error:
`TypeError: channel.detach is not a function`
This happens due to `MailService.joinChannel()` returning a promise
that is resolved with channel ID, instead of a channel object.
`channel.detach()` becomes `<Number>.detach()`, which is treated
like `undefined()`, hence crash.
closesodoo/odoo#44329
X-original-commit: 710fda06c3b30ff2267fcc1d123f15ce031b6f3a
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>