Removes all deprecations from <= 0.7.0

This does a couple things, and seems like at a glance it's a large MR -- and while it sort of is, I did it as one effort to save some time and complexity in MRs. Here are the things to be aware of and review:

• We remove all deprecations that were introduced prior to and as part of the 0.7 releases.
• We changed from using strings for behavior names to using symbols, which generally simplifies several aspects of the code.
  • This is a possibly breaking change for some. Strings were used because that's how the scientist gem had it implemented, but I think using the string value isn't exactly how gitlab-experiment is intended to be used, so feel ok with making this choice and think it's worth seeing.
  • It's also a fairly easy fix -- we can add a name method to the Variant class that returns the name as a string if we decide to roll this back.
• We moved a couple methods from/to the BaseInterface
  • Deprecated the variant_names method, so moved it out of the documented methods.
  • Moved some methods useful for the middleware out of the main Experiment class.