fix(front): add a required attribute to the text editor and show error message if text empty on blur

Also improve text editor borders

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

src/components/Participation/NewPrivateMessage.vue

@@ -32,6 +32,7 @@
       v-if="currentActor"
       :currentActor="currentActor"
       :placeholder="t('Write a new message')"
+      :required="true"
     />
     <o-notification
       class="my-2"