UP-4148 Update contributing guidance

Improves the `CONTRIBUTING.md` guidance for contributing to the uPortal GitHub repository.

Tracked in JIRA at [UP-4148](https://issues.jasig.org/browse/UP-4148).

You can see the new version rendered [here](https://github.com/apetro/uPortal/blob/update_contributing_guidance/CONTRIBUTING.md).

Improvements:

* Emphasize necessity of ICLA.
* Suggest communicating via uportal-dev@ and uportal-user@ email lists.
* Request that code consider style conventions and uPortal architecture.
* Suggest sharing and collaborating in a public feature branch in advance of pull request.
* Adjust feature branch naming convention to include indication of topic beyond just JIRA identifier.
* Add guidance on Pull Request naming and on linking the JIRA from the PR.
* Standardize on "JIRA" vs "Jira"
* Fix "inclusing" typo and "commiting" typo.
* Remove extraneous within-paragraph newlines. Add newlines after section headings.