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ARROW-5526: [GitHub] Add more prominent notice to ISSUE_TEMPLATE.md to direct bug reports to JIRA

This will hopefully help grab the attention of issue reporters to direct them to JIRA better than the current text.

Author: Wes McKinney <wesm+git@apache.org>

Closes #4536 from wesm/ARROW-5526 and squashes the following commits:

378e15990 <Wes McKinney> Respond to feedback, rewrite second part a bit to be more clear about expectation to use mailing lists for non-issue reports
a397cd392 <Wes McKinney> Add more prominent notice to ISSUE_TEMPLATE.md to direct bug reports to JIRA