UX: Improve composer presence-display positioning (#15235)

Previously the discourse-presence plugin was using a `position: absolute` hack to display the 'replying...' users in the top right of the composer. This commit adds a more suitable plugin outlet, and updates the discourse-presence styling so it slots into the flex-box layout at the top of the composer

app/assets/javascripts/discourse/app/templates/composer.hbs

@@ -44,6 +44,9 @@
                 {{/if}}
               </div>
             {{/unless}}
+
+            {{plugin-outlet name="before-composer-controls" args=(hash model=model) tagName="" connectorTagName=""}}
+
             {{composer-toggles
               composeState=model.composeState
               showToolbar=showToolbar