Create welcomebot messages for all Plugin:* channels

Description

The request is to use `/welcomebot set_channel_welcome` for each `Plugin:*` channel

Needs spec to determine exact wording and what to include in the message

Requires a merge of this PR first:

https://github.com/mattermost/mattermost-plugin-welcomebot/pull/31#issuecomment-603819916

QA Test Steps

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Mana

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Assignee

Jason Frerich

QA Assignee

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Reporter

Jason Frerich

Epic Link

Fix versions

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Mattermost Team

Integrations

Sprint

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