[15.0] [mail] search panel is missing in discuss app

[mschulz-asphericon]

Impacted versions: Odoo15

Steps to reproduce: Open the discuss app and any chat or channel

Current behavior: I can only see some messages, but it is not possible to search the messages

Expected behavior: There should be a search bar (or any search at all) to look for specific messages. Especially for the history this feature (in v14) used to be very useful.

Video/Screenshot link (optional):
Odoo15:

Odoo14:

Support ticket number submitted via odoo.com/help (optional):