added urlmap attribute and attempted to add urlmap parameter to existing...

... geocode callback function
I noticed this isn't going to show up as the input's label.

We localize all our strings into other languages, and the way we do this is by adding it to a file with all our strings, without it, editable labels appear as empty strings.

Example: https://github.com/mozilla/popcorn.webmaker.org/blob/master/locale/en_US/popcorn.webmaker.org.json#L117

That line connects all string values to their translated versions, in the above case, it's the English version. By adding an english string to the English version, it means it can then be translated to all our other languages and easily plugged in.

So if you add a line in https://github.com/mozilla/popcorn.webmaker.org/blob/master/locale/en_US/popcorn.webmaker.org.json#L117 like:

```
  "Map URL": "Map URL",
```
It is in alphabetical order, so you'll want to add it right after line 117.