feat(editor): Underline markup for posts+comments

We had tiptap's underline and never used it. A user has asked for it, so
why not implement it right away?

Suggested by `@raimundo@human-connection.social`:
https://human-connection.social/post/612bf465-91ce-4be8-906e-3bde63617346/hcsocial-dein-netzwerk-news-mit-dennis-and-robert-1#commentId-b864d04f-1e8c-48c4-9ed0-98aefaed1c02

backend/src/middleware/xssMiddleware.js

@@ -20,6 +20,7 @@ function clean(dirty) {
       'hr',
       'b',
       'i',
+      'u',
       'em',
       'strong',
       'a',

webapp/assets/_new/icons/svgs/underline.svg

@@ -0,0 +1,5 @@
+<!-- Generated by IcoMoon.io -->
+<svg version="1.1" xmlns="http://www.w3.org/2000/svg" width="32" height="32" viewBox="0 0 32 32">
+<title>underline</title>
+<path d="M7 4h2v12c0 3.37 2.63 6 6 6s6-2.63 6-6v-12h2v12c0 4.43-3.57 8-8 8s-8-3.57-8-8v-12zM5 26h20v2h-20v-2z"></path>
+</svg>

webapp/components/Editor/MenuBar.vue

@@ -5,6 +5,12 @@
 
       <menu-bar-button :isActive="isActive.italic()" :onClick="commands.italic" icon="italic" />
 
+      <menu-bar-button
+        :isActive="isActive.underline()"
+        :onClick="commands.underline"
+        icon="underline"
+      />
+
       <menu-bar-button
         ref="linkButton"
         :isActive="isActive.link()"