[MOV] web, project: make cover image generic in kanban

Previously, we could only set cover image on the kanban view of project
task. This commit makes the feature generic so now cover image can be
added to any kanban view.

Technical notes:
- field used to display the cover image has to be set as 'data-field'
  attribute on the kanban dropdown's  menu related to cover image
- developer will have to take care while designing kanban views with
  cover image ability in such a way that it does not break the design

Task 32108

closes odoo/odoo#31731

Signed-off-by: Martin Geubelle (mge) <mge@openerp.com>