Single analytics file for all builders

[davidfischer]

This pull request breaks out the analytics file readthedocs-dynamic-include.js which is typically created with each docs build into a separate file which will live at https://media.readthedocs.org/javascript/readthedocs-analytics.js.

• This PR only removes readthedocs-dynamic-include.js only for mkdocs. Referencing the same analytics file will be done for sphinx builds as well but that change will be in https://github.com/rtfd/readthedocs-sphinx-ext (probably an update on Remove dynamic include file readthedocs-sphinx-ext#33)
• This PR sets the groundwork for both mkdocs and sphinx to use the same analytics file rather than having any change to analytics require simultaneous corresponding changes across multiple repos.
• If we ever swap out Google Analytics, that could be done server side and we would immediately see analytics with an alternative system rather than waiting for projects to be rebuilt.
• This PR also puts some of my (our?) thoughts on Google Analytics to paper as this isn't an uncommon question we get from privacy conscious users.
• This will result in a minor performance improvement because multiple project docs will use the same readthedocs-analytics.js file which is cached (albeit only for a week with our current settings).

[ericholscher]

Think this is a great direction. Having the ability to change this stuff dynamically in the future is a good first step towards another approach when we choose to move forward with something else.

[ericholscher]

👍