Rename username to discordUsername in DiscordAppeal and update references for clarity.

backend/src/main/resources/templates/discord-appeal-email.html

@@ -96,13 +96,13 @@
     <main>
         <img id="header-img" src="https://beta.alttd.com/public/img/logos/logo.png" alt="The Altitude Minecraft Server" height="159"
              width="275">
-        <h1 style="text-align: center;" th:text="'Appeal by ' + ${appeal.username}">Appeal by Username</h1>
+        <h1 style="text-align: center;" th:text="'Appeal by ' + ${appeal.discordUsername}">Appeal by Username</h1>
         <section class="columnSection">
             <div class="columnContainer">
                 <div>
                     <h2>User information</h2>
                     <ul>
-                        <li><strong>Username:</strong> <span th:text="${appeal.username}">username</span></li>
+                        <li><strong>Username:</strong> <span th:text="${appeal.discordUsername}">username</span></li>
                         <li><strong>UUID:</strong> <span th:text="${appeal.uuid}">uuid</span></li>
                         <li><strong>Email:</strong> <span th:text="${appeal.email}">email</span></li>
                         <li><strong>Submitted at:</strong> <span th:text="${createdAt}">date</span></li>