Mattermost-server: Include '@username' in matterbuild's cutplugin slashcommand response

[mattermod]

When matterbuild’s cutplugin slash command is invoked, the command response does not include the user who triggers the command, which sometimes leads to confusion in the channel as to who invoked the command. Thus the proposal is to include @username in the responses for better visibility.

The POST request that hits the matterbuild service contains the username of the person who triggered the command. All there is required is to extract that data and include it in the command responses.

POST Request contains

channel_id=jux16pkewjrkfj3ehep1psxyxc&
channel_name=town-square&
command=%2Ftest&
response_url=http%3A%2F%2Flocalhost%3A8065%2Fhooks%2Fcommands%2Fxbrkb8p393gjpq5cawei7npije&
team_domain=test&
team_id=carya1qs77bemjup96ff538snh&
text=asd&
token=okwexkjpe7ygb8eq1ww58t483w&
user_id=aoa1agao6t8fmx3ikt1j9w5ybw&
user_name=somename
channel_mentions=\<"saepe-5", "aut-8"]
channel*mentions*ids=\["r3j6sby343fpfdxcbwqg95rfsa", "ehjj46yk7ifptr5bpfb966s6mc"]
user_mentions=\["aaron.peterson", "aaron.medina"]
user*mentions*ids=\["q5s3b7xzgprp5eid8h66j9epsy", "czwmumrmw7dfxecww7qibkkoor">

More details: https://docs.mattermost.com/developer/slash-commands.html#custom-slash-command

Messages to update:
https://github.com/mattermost/matterbuild/blob/master/server/server.go#L446
https://github.com/mattermost/matterbuild/blob/master/server/server.go#L491

Matterbuild repo: https://github.com/mattermost/matterbuild/

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JIRA: https://mattermost.atlassian.net/browse/MM-26990

[utkuufuk]

I'm willing to work on this, already opened a PR: https://github.com/mattermost/matterbuild/pull/46