Switch to stock counterpart

We no longer need the custom counterpart. The things that were
changed in it and reasosn we no lomger need them are:

 1. set separator - this can be done with standard counterpart
 2. ES6 port: this was done to allow importing as _t, but is
    unnecessary with the _t wrapper function in react-sdk
    (although also seems to work fine with
    `import _t from 'counterpart';`)
 3. Fallback to key name: unnecessary with
    `setMissingEntryGenerator` in 0.18, but we've changed to just
    using a fallback locale anyway since No code missing en
    translations should ever make it to the devbelop branch.
 4. Logging on missing translations. Unnecessary with
    `onTranslationNotFound`, but now not used anyway because we
    let missing translations show up in the UI as
    'missing translation'

Bulk changes to imports coming in separate commit for easy reading.

package.json

@@ -50,7 +50,7 @@
     "browser-request": "^0.3.3",
     "classnames": "^2.1.2",
     "commonmark": "^0.27.0",
-    "counterpart-riot": "^0.18.1",
+    "counterpart": "^0.18.0",
     "draft-js": "^0.8.1",
     "draft-js-export-html": "^0.5.0",
     "draft-js-export-markdown": "^0.2.0",