Add html landmarks and update names

What does this MR do?

This MR adds html landmarks and updates their names. This provides easier navigation on a page for screen readers since they can jump between landmarks, and adds meaningful names to them.

Screenshots (strongly suggested)

before	after
You can only jump between the menubar, breadcrumbs, and right sidebar, and they are not well described	You can jump between the menubar, left and right sidebars, breadcrumbs, and main content, and they also have descriptive names
Screen_Recording_2021-01-25_at_4.11.13_pm	Screen_Recording_2021-01-25_at_4.12.22_pm

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team