Register and use config value jquery_force_enable

[benjaoming]

We'll give extension maintainers and Sphinx project owners a definitive way to use the bundled jquery version regardless of the Sphinx version. We might add other config values if necessary.

See: https://github.com/readthedocs/readthedocs.org/pull/9665/files#diff-905da7b9060a47daa15865a5d6a217dad9102274b663ad7778a6b41c3984df97R82