CRM16421 CRM 17633: Changes to support WP in it's own directory. Allow for civicrn...

CRM16421 CRM 17633: Changes to support WP in it's own directory. Allow for civicrn.settings.extra.php for WP. Create template for civicrm.settings.extra.php. Update install/civicrm.php for all needed params for different common install scenarios. Fix linting issues in civicrm.php.
CRM 16421 CRM 17633 - update CRM_Utils_System_WordPress to allow for common install configurations

CRM-16421 - Convert constants to `$civicrm_paths`

Following up on the discussion from
[#10513](https://github.com/civicrm/civicrm-core/pull/10513), this converts
the proposed constants `CIVICRM_UF_WP_BASEURL` and `CIVICRM_UF_ADMINURL` to
variables in the `Paths` system.

A few benefits:
 * Reduces code duplication between `civicrm.php` and `WordPress.php`.
 * Can construct sub-paths with prettier notation (`Civi::paths()->getUrl('[wp.frontend]/foo.txt')`)
 * Has options to output relative or absolute URLs
 * Can expand on `Paths` to provide more inspection/validation

Notes:

 * `CIVICRM_UF_WP_BASEURL` => `wp.frontend.base`
 * `CIVICRM_UF_ADMINURL` => `wp.backend.base`

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* CRM-16421: Work to get CiviCRM for WordPress in WordPress' official Repository
  https://issues.civicrm.org/jira/browse/CRM-16421

CRM-16421 - Assimilate `civicrm.settings.extra.php` into `civicrm.settings.php`

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* CRM-16421: Work to get CiviCRM for WordPress in WordPress' official Repository
  https://issues.civicrm.org/jira/browse/CRM-16421

CRM-17633 merge current master changes to civicrm.settings.php.template