Before this commit, the "add reaction" button was available even when
the thread is not persisted. This is wrong since those messages are only
temporary thus not known by the server: a crash would occur when trying
to add a reaction to such messages.
This commit disables reactions for temporary messages.
Steps to reproduce the issue:
- Go to the `/contactus` page
- Open a chat with the welcome bot
- Try to add a reaction to its first message
- An error modal appears
task-3603577
closesodoo/odoo#142718
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>