accessibility updates

[sscott710]

Sustainability changes:

• updated GMU logo
• added link to RRCHNM accessibility statement in footer

Accessibility changes (more detailed description on the issues found in the accessibility-audit repo, issue 8):

• slightly darkened link text color to meet color constrast of 4.5:1 minimum
• added title attributes to embedded iframes
• made ORCID logo links consistent (aria-label="ORCID profile for [Name]")
• fixed heading hierarchy on submissions and about pages to utilize h2
• fixed inline <span style="color:orange">Group #2</span> in article v03-01 ("Bridge Between Two Worlds" because it failed constrast
• gave raw URL string context for screen readers
• gave footnote back-reference links aria description
• changed homepage <title> to reduce redundancy
• added unique meta descriptions to pages
• added alt text to ORCID icons
• in v03-01, changed the Group 1 from red to #e60000 (slightly darker shade) and fixed the tables (not rendering in markdown due to typo)
• in 404.md, fixed the color constrast of the "return to homepage" button
• wrote alt text for 18 images across v08-06 (4) and v08-07 (14)
• 08-03:46–97 — a 20-row itinerary table has no header, added this
• layouts/index.html, _default/list.html, _default/term.html render /essays/, /volume/2020/ etc. with zero

  — the site title only becomes an h1 on the homepage. Fixed this

• added lang attribute to foreign language text in (v08-03, v08-04, v08-07
• removed <h3> for button styling on 404.md