Fix custom page icon size for openHAB icons (#2445)

Regression from #2340.

Reported on the community:
https://community.openhab.org/t/openhab-4-2-milestone-discussion/154316/2?u=florian-h05.

Signed-off-by: Florian Hotze <florianh_dev@icloud.com>

bundles/org.openhab.ui/web/src/components/app.vue

@@ -27,7 +27,7 @@
                         :class="{ currentsection: currentUrl === '/page/' + page.uid || currentUrl.indexOf('/page/' + page.uid + '/') === 0 }"
                         :link="'/page/' + page.uid"
                         :title="page.config.label" view=".view-main" panel-close>
-            <oh-icon slot="media" :icon="pageIcon(page)" />
+            <oh-icon slot="media" :icon="pageIcon(page)" height="18" width="18" />
           </f7-list-item>
         </f7-list>
         <f7-block-title v-if="$store.getters.isAdmin" v-t="'sidebar.administration'" />