registration: Fix handling of MessagesDirs array and add tests

registration: Fix handling of MessagesDirs array and add tests

Previously the code was designed to handle:
  "MessagesDirs": {
    "FooBar": "i18n"
  }

However, it can also be an array, and some extensions (VisualEditor)
use it like:

  "MessagesDirs": {
    "FooBar": [
      "i18n",
      "also-i18n"
    ]
  }

This properly handles both strings and arrays and adds tests to verify
the behavior.

Change-Id: Iff1523b86f754cac1f5b8d822d4324c5fbfc1a50