Handle accessibility alerts on help page

[luistoptal]

Pull request for issue: #1330

Review a11y issues in http://gigadb.gigasciencejournal.com/site/help and fix them whenever possible

How to test?

Navigate to http://gigadb.gigasciencejournal.com/site/help, run WAVE audit https://wave.webaim.org/ and check that issues have been handled in some way

How have functionalities been implemented?

• I removed all links to the GigaDB website as we are already in that website and so they don't make sense
• I removed the linked urls in the curl commands as these are just examples of curl requests and the user should not follow those links
• I extended the screen reader only text on the links to xsl and pdf files
• Tweaked the footer so that the color contrast issue no longer appears in the WAVE audit (it didn't matter from a UX prespective, but removing the false positive on the common footer improves the DX)
• Tweaked footer links so that they have hover effect

Any issues with implementation?

See #1330 (comment) for details on the challenges on optimizing a11y for this page