Bug 1364894 - Ensure revision.txt exists in development too

Currently `revision.txt` only exists on Heroku, since it's generated
by the Heroku-only `post_compile` script, just prior to `yarn build`.

However this means:
* HTTP 404s of `revision.txt` are seen in the browser console when
  developing locally, which gives the appearance of something being
  broken, even though it's not.
* when we convert the wildcard `CopyPlugin` rule to an explicit list
  of files to copy (in a later commit), it will cause errors when
  building locally, since `CopyPlugin` expects all declared files
  to exist.

Adding a placeholder file prevents both of the above.

ui/revision.txt

@@ -0,0 +1,4 @@
+In production this file will be overwritten with the Git revision, for use by:
+* the UI's "New version of Treeherder has been deployed" notification
+* the changelog URL calculated by `bin/pre_deploy` that's sent to New Relic
+* the What's Deployed tool