- Splits existing single `SpammableActions` module into three separate
  cohesive, loosely-coupled, and well-named new modules:
  - `SpammableActions::CaptchaCheck::JsonFormatActionsSupport`
  - `SpammableActions::CaptchaCheck::HtmlFormatActionsSupport`
  - `SpammableActions::AkismetMarkAsSpamAction`
- Updates and adds test coverage for the new modules.
- Moves responsibility for redirects out of shared modules and back into
   controllers.
- For clarity, inlines responsibility for determining whether a given
  `issueable` should be spam checked into `IssuableActions#update`,
  including preserving existing inconsistent behavior with regards to
  redirects. Comments added to explain what is happening.
- Renames `MarkAsSpamService` to `AkismetMarkAsSpamService` to make it
  clear that it is specific to Akismet.
- Refactors and DRYs up other related areas of logic