add a few methods to help interacting with CI status

related to #6

this adds pipeline_status which returns the status of CI for the current MR (success, failed or running), as well as some methods to know when the bot interacted with a MR. These are useful for #6 as they allow the bot to not simply spam "I'm not assigning someone because CI is failing" every hour.

These methods are currently not used. Below is a sample policy that would assign only when CI succeed, and print a message when it's not assigning due to CI error, limiting this message to at most one per CI failure, and at most one every two days.

resource_rules:
  # These rules only applies to Merge Requests.
  merge_requests:
    rules:
      # We give each rule a name that let's us find it in the logs later.
      - name: Assign reviewers to all MR's with no reviewers and passing CI.
        # All conditions must evaluate to true before the actions are going to
        # be executed.
        conditions:
          # See src/tor.rb for how this function is defined. Return true
          # iff the given MR have a reviewer assigned, otherwise false.
          ruby: |
            !has_reviewer? && pipeline_status == "success"
          # We only look at open tickets.
          state: opened
        # Execute all of the actions if all of the above conditions are satisfied.
        actions:
          # We use Gitlab's Quick Actions to randomly assign the MR to
          # someone in the network team who is not the author of the MR.
          comment: |
            /assign_reviewer @#{ random_reviewer(["eta", "Diziet", "nickm"]) }
      - name: Warn failing CI.
        conditions:
          ruby: >
            !has_reviewer? && pipeline_status == "failed" &&
            bot_last_interacted < pipeline_end_time &&
            bot_last_interacted < (DateTime.now - 2)
          state: opened
        actions:
          comment: |
            it looks like CI is failing :confused: