Our hosting service support was introduced fairly early in the life of

Review Board, and the base support has outgrown the scattered files

they've lived in. With some upcoming work on the horizon to both remove

old code and to add new support for SSL certificates and eventually

deeper bug tracker integration, it's time to clean up a lot of this.
This change splits reviewboard.hostingsvcs.service into a series of

modules in reviewboard.hostingsvcs.base, as well as moving

reviewboard.hostingsvcs.utils.paginator and

reviewboard.hostingsvcs.{forms,repository} into there.
To keep this reviewable, there are no changes to these files aside from

some "Version Changed" docs in classes detailing the move, and new

module docstrings. Currently, nothing is marked as deprecated with a

warning, and all prior imports will continue to work.
Follow-up changes will rename some of the new classes to better signify

base classes and to add typing. For Review Board 6, the old locations

will not be deprecated, leaving that to Review Board 7. This is to avoid

introducing major deprecations this late in the development cycle.

All unit tests pass.