Add links to community platforms in the support dialog

[saif-el]

We now have a Slack channel and Discourse category for allowing users to post their inquiries regarding El-MAVEN. It makes sense to put these two links inside the application's support section itself.

We should remove the link to the mailing list (which we are phasing out in favor of the other two):

... and instead add a note saying:

"Contact tech support on Slack or Discourse"