update DisabledContentView to support reporting broken sites

[antonok-edm]

Changes

DisabledContentView now has a vertical layout to support the "Report a broken site" button in accordance with mockups in brave/brave-browser#4262.

The default dark and shields dark themes were also updated to support showing the new report button in dark mode.

Required for brave/brave-core#3420.

Test plan

Storybook was updated to reflect the new DisabledContentView and its contents.

Link / storybook path to visual changes

• http://localhost:9091/?path=/story/feature-components-shields--panel

Integration

• Does this contain changes to src/components or src/

  • Will you publish to npm immediately after this PR, or wait until sometime in the future?
  • Incompatible API change to something existing (major version increase)
  • Adding new backwards-compatible functionality? (minor version increase)
  • Fixing a bug backwards-compatibly? (patch version increase)

• Does this contain changes to src/features for brave-core?

  • Are there non backwards-compatible changes required for brave-core? Do not merge until brave-core PR is approvable. Link to brave-core PR: Add webcompat issue reporting dialog brave-core#3420
  • Will you create brave-core PR to update to this commit after it is merged?
  • Wants uplift to brave-core feature branch?
    • When uplift-approved, merge to brave-core-0.VV.x feature branch
    • Create additional brave-core PRs for each feature branch to update commit

[antonok-edm]

@petemill @cezaraugusto looks like as of brave/brave-core@e76aa02, all the shields-specific UI components are now in brave-core. I've just moved the components from here into the brave-core PR, is it still important to get these changes in brave-ui?

On a somewhat-related note, I did add one new theme variable to the brave-dark theme to update the color of the "Report a broken site" button on the dark-mode Shields panel - and that button would theoretically be useful in the modal dialog itself - so I will still depend on a brave-ui PR anyways.

Edit: Theme requirements for the webcompat reporter dialog PR are now in #542.

[cezaraugusto]

sorry for taking so long to review this. LGTM