[FIX] portal_rating: fix invisible rating modal title

How to reproduce:
1. Go to website slides.
2. Open any course.
3. Add review or Edit the review.

The title of the modal should be visible but it is not because the text has the
same color as the background (white).

Technical note: the modal is defined in a dark zone where the text color is
white. The modal title uses the h5 tags that automatically inherit its color
from its parent, so the title is white as well. To solve the problem we add the
class "bg-white" on the modal-content div to indicate that the background of
the modal is white so that the color of the h5 has a good contrast on a white
background (here black). We also removed the CSS rule for
"o_rating_popup_composer_label" as it was not being applied.

Task-3640496

closes odoo/odoo#149405

X-original-commit: 0f158f4c3b51a4caa20dda337d13e905c99540fd
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/portal_rating/static/src/scss/portal_rating.scss

@@ -81,7 +81,3 @@ $o-w-rating-star-color: #FACC2E;
         margin-right: 10px;
     }
 }
-
-.o_rating_popup_composer_label {
-    color: color-contrast(white);
-}

addons/portal_rating/static/src/xml/portal_rating_composer.xml

@@ -7,7 +7,7 @@
     <t t-name="portal_rating.PopupComposer">
         <div t-if="widget.options['display_composer']" class="modal fade" id="ratingpopupcomposer" tabindex="-1" role="dialog" aria-labelledby="ratingpopupcomposerlabel" aria-hidden="true">
             <div class="modal-dialog" role="document">
-                <div class="modal-content">
+                <div class="modal-content bg-white">
                     <div class="modal-header">
                         <h5 class="modal-title o_rating_popup_composer_label" id="ratingpopupcomposerlabel">
                             <t t-if="widget.options['default_message_id']">