Add TypeScript definitions for better guardrails

Problem: We have TypeScript enabled on the project but have been missing
lots of definition files, so there are like 500 errors when you enable
`--noImplicitAny`.

Solution: Add definitions and fix the bugs that they point out. This
reduces the number of errors with `--noImplicitAny` to 285.

src/views/markdown.js

@@ -53,5 +53,6 @@ const toUrl = (mentions) => {
 module.exports = (input, mentions = []) =>
   md.block(input, {
     toUrl: toUrl(mentions),
+    /** @param character {string} */
     emoji: (character) => span({ class: "emoji" }, character).outerHTML,
   });