Rename `homepage2` to `ui`

Just a more sensible name since it's not a homepage of any sort.

Test plan: Flow passes, `yarn build2` and `yarn start2` both succeed.
Also: `git grep homepage2` gets no hits.

config/paths.js

@@ -15,9 +15,9 @@ module.exports = {
   appBuild: resolveApp("build"),
   appBuild2: resolveApp("build2"),
   appIndexJs: resolveApp("src/homepage/index.js"),
-  appIndexJs2: resolveApp("src/homepage2/index.js"),
+  appIndexJs2: resolveApp("src/ui/index.js"),
   appServerSideRenderingIndexJs: resolveApp("src/homepage/server.js"),
-  appServerSideRenderingIndexJs2: resolveApp("src/homepage2/server.js"),
+  appServerSideRenderingIndexJs2: resolveApp("src/ui/server.js"),
   appPackageJson: resolveApp("package.json"),
   appSrc: resolveApp("src"),
   yarnLockFile: resolveApp("yarn.lock"),