Matrix: Setup bot, add PIN verification

PIN is verified but not used currently. Works a little different than
the others, you input your matrix user ID and then the PIN is sent to
you. The bot doesn't support E2EE, so the bot being the first one to
message ensures the chat is unencrypted.

html/form-base.html

@@ -22,6 +22,9 @@
     window.discordPIN = "{{ .discordPIN }}";
     window.discordInviteLink = {{ .discordInviteLink }};
     window.discordServerName = "{{ .discordServerName }}";
+    window.matrixEnabled = {{ .matrixEnabled }};
+    window.matrixRequired = {{ .matrixRequired }};
+    window.matrixUserID = "{{ .matrixUser }}";
 </script>
 <script src="js/form.js" type="module"></script>
 {{ end }}