Fix accessibility errors on dataset page

[luistoptal]

Pull request for issue: #1329

Minimize a11y issues in dataset page, eg. http://gigadb.gigasciencejournal.com/dataset/100006

How to test?

Run WAVE audit in any dataset page and confirm there are no errors

As for the "alerts" they are caused by duplicate file links in the files table which we opted to leave untouched, because it is not clear to us whether this is a requested feature or not

How have functionalities been implemented?

• Override background color of bootstrap stripe table to use one of the colors defined in our styles, also override text color on such context to make sure it is accessible
• Override color of + / - button to make it accessible
• Update color styles of sr-only text to minimize accessibility errors, these errors do not affect the UX but are annoying when running a11y audits

Any issues with implementation?

--

Any changes to automated tests?

--