Enhance Atom feeds - Githubissues

Offer a listing of Atom feeds and prepopulate supported categories.

Contributor Checklist

[x] Check "Allow edit from maintainers" option in pull request so that additional changes can be pushed by the Bug Alert team if needed.
[x] I have performed a self-review of my own code.
[x] I have commented my code, particularly in hard-to-understand areas.
[x] My changes generate no new warnings.
[x] I have run Pelican locally to ensure my changes have not disrupted the look, feel, or functionality of the site.
[x] I have checked my code and corrected any misspellings.
[x] I have added a pull request description which details the nature of this change.

Leave the following intact, which notifies our volunteer team: @BugAlertDotOrg/volunteer-team