Update metrics: track forum post tags, message channel category

[vcarl]

The Star Helper admin work has shown me that we could more effectively use our metrics to drive activity-based segments of the community if we can segment activity by channel group, i.e. by asking for a chart of the top 25 posters by message volume in the "Need Help" channel category, vs pulling out individual channel IDs to build the same chart.

Similarly, we could use channel tags to monitor usage of different tags. Right now we don't make much use of forums, but we're exploring a change to help channels that would more strongly emphasize them.

[what-the-diff]

PR Summary

• Addition of new function and event handlers in the stats feature
  The changes in the 'stats' feature now allow for applied tags on threads to be mapped to their corresponding tag names. This has been implemented through the addition of a new function named mapAppliedTagsToTagNames. The feature also includes a new constant titled postTagged and two new event handlers threadUpdate and threadCreate. These event handlers will activate the postTagged event whenever an update or creation action is performed on threads with applied tags.

• Implementation of new function in the discord helper
  The 'discord' helper now has an efficient way to map applied tags on threads to their respective tag names. This is facilitated by the addition of a new function mapAppliedTagsToTagNames.

• Addition of explanatory comment in the sets helper code set
  An instructive comment has been included in the 'sets' helper. This comment will aid in understanding that the difference function is meant to produce a list containing items present in Set A but missing in Set B.

[ghost]

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