[A11y] SiteImprove a11y best practice violations on timeline view page

[Dananji]

Description

The timeline view page including the timeline settings modal at https://timeliner.dlib.indiana.edu has the following a11y best practices violations when scanned with SiteImprove,

Message	Relevant HTML element	Notes
Page does not start with a level 1 heading	Timeline view page	A level 1 heading <h1> tells the user what the page is about before they decide to navigate through the content. If a page starts with a different heading level, it can cause confusion for the user
Headings are not structured	Current page	One or more heading levels have been skipped
Content missing after heading	<h6> elements for "Video Playback" and "Timeline information" panel titles	Content should appear as expected including between headings of the same level and after the last section or page heading.
Skip to main content link is missing	Current page	The first item reached by keyboard should be a link to the main content of the page

Part of avalonmediasystem/avalon#6435.

Done Looks Like

• SiteImprove doesn't show any best practice violations for all parts of the timeline view page
  • timeline view
  • timeline view with the edit bubble/marker form opened
  • timeline settings modal