Update DocUp posts in Mattermost when GitHub issue for the documentation is resolved

[jwilander]

From @aaronrothschild:

It would be really cool if the plugin could watch and update the "docup" confirmation messages with a direct link to the documentation when they get submitted as a PR. Would Saves a few clicks for people browsing old conversations in MM channels that have been doc'd. But I realize, this would add a lot of complexity.