Include forum and enterprise support links in sidebar menu

[jspaleta]

Goal here is to breadcrumb the Discourse and optionally the Enterprise Support browser reachable resources for user of the web-ui.

We already have a github issues link as a "Feedback" icon and these additions would be companion links to round out the communication points of contact.

Strawman: Include a "Forum" or "Community" icon in the sidebar that goes to our discourse homepage: https://discourse.sensu.io/

When a valid enterprise license is present, also include an icon for "Support" that leads to the appropriate place for enterprise support.