Mattermost-server: Add "Unarchive Channel" option to the channel info screen

[mattermod]

Replace the “Archive Channel” option with an "Unarchive Channel" option in the channel info screen when the channel is archived.

Notes:
1 - To assist with the implementation, review the existing API call for restoring a channel (which is equivalent to unarchiving)
2 - When chosen, the channel should be become active again and added to the channel sidebar for all members in the respective public or private lists depending on type of channel prior to archive.
3 - A system message should be posted to the channel: "\ unarchived this channel."

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JIRA: https://mattermost.atlassian.net/browse/MM-20804

[allenlai18]

Can i work on this?

[enahum]

@allenlai18 absolutely

[umlaeute]

will this be available for the community edition, or only in the payed plans?

[jasonblais]

@umlaeute This will be available in all Mattermost editions in v5.22