Add links to GitHub and Mattermost channel in the IDE

[slemeur]

We should include links in the product in order to let a user being able to engage with the community more easily.

Since we opened the mattermost channel we should add a new entry in the Help menu:

The item "Public Chat" should be below "Community" and point to: https://mattermost.eclipse.org/eclipse/channels/eclipse-che

Also we add "Report Issue" and point to: https://github.com/eclipse/che

[vparfonov]

@slemeur Is this still actual ?

[slemeur]

Yes.