Adds option to disable the tag without any logging side-effect.

Hi @kencochrane,

I'm submitting this PR to fulfil what I think is a convenient feature for developers. It allows the tag to be disabled with one setting in situations such as a developer environment where you may not want to send user information to intercom on every request (particularly if multiple developers are sharing a dev application Id).

It means no change is needed to remove/comment out the tag in templates or the INTERCOM_APPID setting. Initially I'd simply commented out INTERCOM_APPID so that INTERCOM_IS_VALID: False was passed to the template but that has the side effect of logging "INTERCOM_APPID isn't setup correctly in your settings" on every request.

I've implemented it in what I think is the lightest touch on the rest of the code.