Erase the `CONTENT CREATED` activity once an `EDITED_CONTENT` comes

[brecke]

This is a result of #1967

What I'd like to do is to erase the created event for an etherpad/ethercalc document once the correspondent edited event comes. Right now this is get:

• User creates etherpad/ethercalc -> CREATED activity without preview
• User edits etherpad/ethercalc -> EDITED activity with preview

This would be something along the lines of:

    ActivityAPI.postActivity(ctx, activitySeed, err => {
      /**
       * After we post a new EDITED activity we are going to erase the CREATED one
       * This is because it's mostly useless. The CREATED activity is only useful for other
       * people to be able to join and collaborate on a document. Once that is published
       * the first time, the EDITED activity is prevalent and will also have a preview
       * (unlike the CREATED activity)
       */

      const activityStreamId = `${activitySeed.actorResource.resourceId}#activity`;

      // get 10 most recent activities
      const startFromNewest = null;
      getActivities(activityStreamId, startFromNewest, 10, (err, mostRecentActivities) => {
        // then make sure we pick the ones where `oae:activityType:"content-create"`
        forEach(eachActivity => {
          // when we find the one that corresponds to the same `objectResource`
          const isContentCreated = activity => equals('content-create', path(['oae:activityType'], activity));
          const isTheSameContent = activity => equals(path(['object', 'id'], activity), activitySeed.objectResource.id);
          if (both(isContentCreated, isTheSameContent)(eachActivity)) {
            console.log('ERASE THIS GUY');
            // then erase it
          }
        }, mostRecentActivities);
      });
    });
  }
);