Rename `mark_as_ham!` to `execute`

In order to make HamService consistent with Service architecture, rename mark_as_ham! to execute

• @ashmckenzie started a discussion: (+1 comment)

  In a separate MR, WDYT about renaming this (and the implementation) to #execute because the class name is already very specific and also to more closely match the pattern of existing classes under app/services ?