Use cpx to copy olm.js, and add watcher

We are now using `cpx` to copy the emojione files, so we might as well use it
for olm.js, rather than rolling our own - particularly since that makes it easy
to add a watcher for `npm start`.

We have to add a `-L` to the `cpx` invocation, otherwise, if olm is a
symlink (as is useful for development), cpx ends up watching the symlink, which
obviously never changes.

scripts/staticfiles.js

@@ -1,21 +0,0 @@
-#!/usr/bin/env node
-
-// copy static files from node_modules to the vector directory
-//
-
-var fs = require('fs-extra');
-
-function exists(f) {
-    try {
-        fs.statSync(f);
-        return true;
-    } catch(e) {
-        return false;
-    }
-}
-
-const olm = 'node_modules/olm/olm.js';
-if (exists(olm)) {
-    console.log("copy", olm, "-> vector");
-    fs.copySync(olm, 'vector/olm.js');
-}