[MM-128]: Added functionality to add user to all public channel in the team at once

[Kshitij-Katiyar]

Summary

Added functionality to add users to all the team's public channels at once.
This can be achieved by adding * to ChannelsAddedTo in the config.json(as described in readme)

Ticket Link

Fixes #128

What to test

1. Update the config.json file by updating the value '*' in ChannelsAddedTo Field.
2. Add a new user to the team and he should be added to all the public channels of that team.

[nathanaelhoun]

Hello, I needed this functionality and seeing that @Kshitij-Katiyar already had implemented it, I compiled it and gave it a try on my Mattermost v9.11.1 instance.
Worked perfectly, thanks!