Refactor: use "message" instead of "comment" consistently

[bess]

We recently changed our "comments" into "messages." Instead of just changing the external label, let's also refactor the code to use the same language for these objects consistently. Otherwise it's going to make the code more confusing than it needs to be going forward.

Acceptance Criteria

• All places where the code references to "comment" have been changes to "message"

[mccalluc]

Following #837, there is one remaining piece of work:

class WorkActivity < ApplicationRecord
  CHANGES = "CHANGES"
-  COMMENT = "COMMENT"
+  MESSAGE = "COMMENT" # TODO: Migrate existing records to "MESSAGE"; then close #825.

A migration could be created and run to make sure the code in the database matches the code in the UI and the rest of the codebase.

[kelynch]

This appears to be completed.