Mute functioanlity also silences DM from muted user

[DHaussermann]

There is functionality in GutHub to send a DM when a another GitHub user comments on PR or issues you are assigned to even if they do not have @mentions. There is also a mute feature to silence the messages from a specific user(s).

In previous releases - When a user is muted, comments on your PRs and Issues do not trigger a DM. However, if the comment contains an @mention you would get a DM regardless.

It was recently discovered that in master branch and Release bump 2.1, These DMs are also muted.

Steps:

1. Connect test user to GitHub
2. Create and issue in GitHub and assign it to yourself
3. As an alternate GitHub user, comment on the issue with no @mention
4. Note the test user receives the a DM
5. As the test user in Mattermost use the GitHub mute command to mute the user in from step 3.
6. As the alternate GitHub user, comment on the issue on the issue with no @mention
7. Note the test user no longer receives the DM from muted user (this step is just to confirm the correct user is targeted and mute is working)
8. As the alternate GitHub user, comment on the issue on the issue with an @mention Observed: No Dm is received by the test user Expected: @mentions should supersede mute logic and user should receive a DM

[mickmister]

@DHaussermann I think this issue needs to have some UX discussion, as this point is a bit subjective:

Expected: @mentions should supersede mute logic and user should receive a DM

I'm not sure if the feature had this in its original design. I can imagine a spammy bot mentioning a given user a bothersome number of times in quick succession. If the user explicitly mutes a given user, they likely never want notifications from that user.

[hanzei]

@-mentions should not supersede a mute setting. If there are case where a @-mention notification gets through, that would be a bug.

@DHaussermann Could you please check if that case exists?