Add a delete button for chats in the UI

[pmeier]

Feature description

Per title.

Value and/or benefit

In #62 we agreed to have a proper chat deletion feature. This was implemented for the REST API in #67. However, for time reasons, we never got around implementing it for the UI.

There is already some design for it:

The comment on the cancel button is from @smeragoel and reads

Can't use our primary here because it will be too close to orange

@smeragoel What would be the right color to use here? Our blue? The purple used in the design isn't defined anywhere AFAIK.

Anything else?

No response