[FIX] web_editor, *: make the disabled remove form field button red too

*: website

The fields of a form that are mandatory because of the model they need
to be used in cannot be removed. When such a field is selected, the
remove button displays a tooltip giving this information.
Disabling the overlay button is done by removing the specific class -
which makes the event selector not linked to the button, but also
impacts the applied CSS rules.

In stable, this commit only updates the CSS to keep the look-and-feel of
the remove button when it is disabled.

In master, this commit will add an `o_disabled` CSS class upon disabling
buttons and adapt the event selectors accordingly - making it possible
to not impact any `oe_snippet_*`-specific CSS rule.

Steps to reproduce:
- Drop a "Form" block.
- Select the "Phone Number" field.
- The overlay's delete icon has a reddish background.
- Select the "Your Email" field.
=> The overlay's delete icon had a dark gray background instead of a
reddish one.

task-2950433

closes odoo/odoo#107574

X-original-commit: 166dca8a
Signed-off-by: Romain Derie (rde) <rde@odoo.com>
Signed-off-by: Benoit Socias (bso) <bso@odoo.com>