Fix for skipping 'open.php' replacement in non standard drupal install

[kainuk]

Before

Urls with extern/open.php are not replaced when civicrm is not installed in the sites/all/modules directory. The effects is that open clicks are not sent not to the proxy server but but direct to civicrm and lost.

After.

The urls are replaced.

Technical

• Use $config->userFrameworkResourceURL instead of the literal path.
• Maybe the code is nearly ready for WordPress or Joomla

Background

What this bug makes a bit complicated to find is that a tester often has direct access to the civicrm installation, so his click registers.

[bjendres]

Thanks @kainuk! @jensschuppe does this look good to you?

[jensschuppe]

I'm unfamiliar with what this code actually does, but using CMS-independent URLs seems very reasonable. There's also a sites/default/files thing in there. Should this also be replaced, @kainuk?

[kainuk]

The code replaces an URL that leads to CiviCRM with one that leads to the Proxy

Replacing sites/default/files makes sense because that works only for drupal. Not sure it that's enough to make in CMS independent. This PR solves a problem for a Drupal customer.