[IMP] website_slide: add a card description independent from short description

The Channel model contains the two following fields: `description` and
`description_html`. The former is a short plain-text description that can be
filled in the creation form. The latter is a long html-formatted description.

Before this commit:

The description displayed on the card of the course is the `description` field.

After this commit:

There is an additional `description_short` field, which is initialized to the
same value as the `description` field. If the `description` field is modified
the `description_short` reflects the modifications. This happens however only
if the user didn't modify the `description_short` field manually. If it is
modified to another value manually, the link is broken.

Description short is now used only on the course page, below title, in the
header. Description short is used in kanban card displayed in homepage.
Html description is used as a long course description on course page.

Beside, in the creation form of a course, the description textarea is now a
one-line input field to reflect the fact that it's supposed to be a relatively
short description.

Task ID: 2026165
PR #39388

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>