[FIX] mail: make <hr/> visible

Before this commit, the <hr/> in Discuss and Chatter
were not visible.

These <hr/> had no height but had a border. The border showed the
separator.

With the redesign of web client, <hr/> now have opacity of 25%.
This combined with borders being lighter made these separators
barely visible.

This commit removes border on these <hr>, which makes them use
their default visual, which is the same as a border.

closes odoo/odoo#121378

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/mail/static/src/core_ui/thread.xml

@@ -20,9 +20,9 @@
                 <t t-foreach="props.order === 'asc' ? props.thread.nonEmptyMessages : [...props.thread.nonEmptyMessages].reverse()" t-as="msg" t-key="msg.id">
                     <t t-if="msg.dateDay !== currentDay">
                         <div class="o-mail-Thread-date d-flex align-items-center fw-bolder pt-4">
-                            <hr class="flex-grow-1 border-top"/>
+                            <hr class="flex-grow-1"/>
                             <span class="px-3"><t t-esc="msg.dateDay"/></span>
-                            <hr class="flex-grow-1 border-top"/>
+                            <hr class="flex-grow-1"/>
                         </div>
                         <t t-set="currentDay" t-value="msg.dateDay"/>
                     </t>

addons/mail/static/src/web/chatter.xml

@@ -128,13 +128,13 @@
 <t t-name="mail.ActivityList" owl="1">
     <div class="o-mail-ActivityList">
         <div class="d-flex pt-4 cursor-pointer fw-bolder" t-on-click="toggleActivities">
-            <hr class="flex-grow-1 border-top"/>
-            <div class="px-3">
+            <hr class="flex-grow-1"/>
+            <div class="d-flex align-items-center px-3">
                 <i class="fa fa-fw" t-att-class="state.showActivities ? 'fa-caret-down' : 'fa-caret-right'"/>
                 Planned Activities
                 <span t-if="!state.showActivities" class="badge text-bg-success"><t t-esc="activities.length"/></span>
             </div>
-            <hr class="flex-grow-1 border-top"/>
+            <hr class="flex-grow-1"/>
         </div>
         <t t-if="state.showActivities">
             <t t-foreach="activities" t-as="activity" t-key="activity.id">

addons/mail/static/src/web/discuss_app/sidebar.xml

@@ -8,7 +8,7 @@
                 Start a meeting
             </button>
         </div>
-        <hr class="w-100 border-top"/>
+        <hr class="w-100"/>
         <div class="d-flex flex-column flex-grow-0">
             <t t-call="mail.Mailbox">
                 <t t-set="mailbox" t-value="store.discuss.inbox"/>
@@ -20,7 +20,7 @@
                 <t t-set="mailbox" t-value="store.discuss.history"/>
             </t>
         </div>
-        <hr class="w-100 border-top"/>
+        <hr class="w-100"/>
         <input t-if="hasQuickSearch" class="form-control mx-4 mb-2 rounded-3 w-auto" placeholder="Quick search..." t-model="state.quickSearchVal"/>
         <t t-call="mail.DiscussCategory">
             <t t-set="category" t-value="store.discuss.channels"/>