Add link for creating a new github issue in this repo under "help" section

[shell1]

link using github icon from help section that goes to https://github.com/mozilla/experimenter/issues/new

When people need to request advanced targeting - they need to know to file a ticket in this repo. Under the feedback link - should have a link for "File targeting criteria request"

common scenarios:

• need a specific release (nightly build, beta version, or dot release) targeted that isn't offered off the version list by default.
• need advanced targeting that isn't available.

┆Issue is synchronized with this Jira Task