Add support to specify the locale of a user in the notification repository

[wspee]

Background

For our upcoming cross promotion campaign we want to be able to show notifications to users that use a specific locale.

What to change

Add support to specify the locale of a user by adding a space separated list of locales.

Notification repository format:

inactive = no

severity = normal
title.en-US = Test
message.en-US = Test
target = extension=adblockpluschrome locale=en-US locale=de

Resulting notification.json:

{
  "notifications": [
    {
      "targets": [{
        "extension": "adblockpluschrome",
        "locales": ["en-US", "de"]
      }],
      "id": "1",
      "message": {
        "en-US": "Test"
      },
      "severity": "normal",
      "title": {
        "en-US": "Test"
      }
    }
  ],
  "version": "201707281538"
}

[trev]

I've updated the suggested format as mentioned in #5460 review.

[kvas]

A commit referencing this issue has landed:
​Issue 5457, 5458 - Added support for locales and blocked total to notification parser...

I guess the commit message ​should have started with Issue 5457, Issue 5458 -. Perhaps we should document this.