add announcement emails

After selecting users in the accounts tab, you can press 'Announce',
then write a subject and message (with markdown), and an email will be
sent to each selected user.

css/base.css

@@ -164,6 +164,7 @@ div.card:contains(section.banner.footer) {
 
 .monospace {
     background-color: inherit; /* so we can use a17t code blocks */
+    font-family: Menlo,Monaco,Consolas,Liberation Mono,Courier New,monospace;
 }
 
 sup.\~critical, .text-critical {