Support use of staging and dev environments for TagManager

[diosmosis]

Currently, Matomo for WordPress explicitly limits the number of available environments to TagManager. Only live is allowed.

From what I understand, this was done for simplicity. The assumption was that most users would not need the other environments, as users with that need would be more likely to use a standalone Matomo instance.

Some users do want the other environments, however, so we should allow a way for them to access and configure them.

• A new setting should be introduced for TagManager that enables/disables the extra environments.
• The INI config that limits the environments (in common.config.ini.php) should be removed and replaced with PHP code that dynamically limits the environments based on the new setting.
• The UI for selecting a TagManager container in the tracking code settings needs to be modified to also allow selecting the environment.