Make JSExtension available through reviewboard.extensions.base.
Make JSExtension available through reviewboard.extensions.base.

This exposes JSExtension through reviewboard.extensions.base so that
extensions can make use of it.

KnowNo/reviewboard,custode/reviewboard,beol/reviewboard,1tush/reviewboard,KnowNo/reviewboard,brennie/reviewboard,chipx86/reviewboard,1tush/reviewboard,sgallagher/reviewboard,sgallagher/reviewboard,sgallagher/reviewboard,beol/reviewboard,brennie/reviewboard,1tush/reviewboard,bkochendorfer/reviewboard,reviewboard/reviewboard,brennie/reviewboard,reviewboard/reviewboard,1tush/reviewboard,reviewboard/reviewboard,davidt/reviewboard,beol/reviewboard,custode/reviewboard,brennie/reviewboard,reviewboard/reviewboard,sgallagher/reviewboard,chipx86/reviewboard,chipx86/reviewboard,davidt/reviewboard,davidt/reviewboard,1tush/reviewboard,1tush/reviewboard,KnowNo/reviewboard,1tush/reviewboard,bkochendorfer/reviewboard,1tush/reviewboard,bkochendorfer/reviewboard,custode/reviewboard,bkochendorfer/reviewboard,custode/reviewboard,davidt/reviewboard,1tush/reviewboard,beol/reviewboard,chipx86/reviewboard,KnowNo/reviewboard