Changeable admin url (for 2.4)

**Fixes**

Issue #702

**What this does**

This PR allows the administration URL to be changed via the `symphony => admin-path` setting in the config.php file. The admin path is set initially in the installation process at the top of the configuration list.

**Dependencies that might change**

This PR contains three other PRs to make it work.

- #1544 is a reversion of a previous fix which caused the installation to fail.
- #1542 is a migration file for Symphony 2.4 **Do not upgrade with this migration as it will change and your install will break**.
- #1541 is a new constant for the physical `/symphony` folder, separating from the pseudo admin path for symphony.