[Accessibility]: Search contexts management ARC checklist issues

Created by: limitedmage

ARC Toolkit

The following ARC toolkit issues exist in the search context management pages:

Headers should not be H1, should be H2 styled as H1, on all pages (list page, view page, and add/edit page)
On list page, <ul> should use <li> instead of <div> for children
On list page, "Filter search context" textbox should have an aria-label
On edit page, Delete button doesn't meet color contrast requirements (should use a danger variant button instead)

All issues should be fixed as noted above

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This issue will be fixed by GitStart. I have assigned the label 'gitstart' and added the assignee 'gitstart-sourcegraph'.