Some specific events on Gitlab issues and merge requests are stored
separately from comments as "resource state events". With this change,
all relevant resource state events are downloaded during issue and merge
request migration, and converted to comments.
This PR also updates the template used to render comments to add support
for migrated comments of these types.
ref: https://docs.gitlab.com/ee/api/resource_state_events.html
(cherry picked from commit 17f170ee3724d8bdf2ddaad4211b12433f78ff0e)
