doc: delete gitter repo and update related docs

[sulheim]

Description of the issue:

With the newly added Discussion feature on GitHub I don't see the need for an additional gitter repository. I therefore suggest that we delete that repository and edit our issue templates / documentation / badges to remove any pointers to gitter.

To Do

• [x] Post on current Gitter chat room that discussions are moved here
• [x] Update readme
• [x] Update issue template

I hereby confirm that:

• [x] This problem persists in the master branch of the repository
• [x] A similar issue does not already exist
• [x] I've considered asking this first in the Gitter chat room

[sulheim]

closed by #125

[edkerk]

FYI, I found out that if the issue is mentioned in the PR to master (and it will be in #122), then it will automatically be closed when merged.