Add PR previews with Netlify (#5787)

It would be preferable to only generate these for UI PRs but Netlify appears to
not have that flexibility. I tried setting up manual deployments in a Travis
environment but gave up the experiment; it could probably eventually work if
deployment failures become a nuisance.

.netlify/state.json

@@ -0,0 +1,3 @@
+{
+  "siteId": "a3e19a5f-a18e-40b7-b943-bd0f96ba354f"
+}

netlify.toml

@@ -0,0 +1,18 @@
+# Netlify is installed under the hc-nomad bot account on GitHub.
+
+[build]
+  # The generated site needs to live at /ui/, so this moves it there after building.
+  command = "ember build && mkdir -p ui-dist/ui && mv dist/* ui-dist/ui/"
+  base = "ui"
+  publish = "ui/ui-dist"
+
+# This ensures that following the link from the PR checks list redirects to the subdirectory.
+[[redirects]]
+  from = "/"
+  to = "/ui/"
+
+# This catches other unknown URLs and lets Ember handle them.
+[[redirects]]
+  from = "/*"
+  to = "/ui/index.html"
+  status = 200