add legacyhandler back (deleted it in 766 oops)

[smonero]

In https://github.com/lyft/atlantis/pull/766/files#top I accidently deleted the legacyHandler functionality. It worked in staging so I assumed it was fine, but apparently not, since some people had problems where the status was reported. This should fix that.

Just to clarify why it is ok to delete the other parts and not the legacyHandler:

• The legacyHandler actually calls the UpdateCombinedCount func that is needed for github updates
• The other parts, like the comment handler and error handler only forwarded things to sns, which is why we don't need them