adds ability to specify plausible domain

[pschocke]

This PR adds support for self hosted plausible instances.

It adds a new config variable to specify a different plausible domain.

It introduces no breaking chances since it has sensitive default values.

This PR also fixes a bug that if you use an other cp domain as /cp all requests fail.

[jackabox]

Thanks for this, looks good - will review this properly next week and hopefully merge!

[ryanmitchell]

Are there any plans on merging this? We use a self-hosted solution and it would be super helpful to have this built into our Statamic installs.

[jackabox]

@ryanmitchell Released 👍🏻