Consolidate assistiveText props under one object [AppLauncherSection, Avatar, Breadcrumb, ButtonStateful, Button, GlobalHeader, GlobalNavigationBarDropdown]

[garygong]

This PR consolidate assistive text props under one assistiveText prop for the following components:

• AppLauncherSection
• Avatar
• Breadcrumb
• ButtonStateful
• Button
• GlobalHeader
• GlobalNavigationBarDropdown

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CONTRIBUTOR checklist (do not remove)

Please complete for every pull request

• First-time contributors should sign the Contributor License Agreement. It's a fancy way of saying that you are giving away your contribution to this project. If you haven't before, wait a few minutes and a bot will comment on this pull request with instructions.
• npm run lint:fix has been run and linting passes.
• Mocha, Jest (Storyshots), and components/component-docs.json CI tests pass (npm test).
• Tests have been added for new props to prevent regressions in the future. See readme.
• Review the appropriate Storybook stories. Open http://localhost:9001/.
• The Accessibility panel of each Storybook story has 0 violations (aXe). Open http://localhost:9001/.
• Review tests are passing in the browser. Open http://localhost:8001/.
• Review markup conforms to SLDS by looking at DOM snapshot strings.

REVIEWER checklist (do not remove)

• TravisCI tests pass. This includes linting, Mocha, Jest, Storyshots, and components/component-docs.json tests.
• Tests have been added for new props to prevent regressions in the future. See readme.
• Review the appropriate Storybook stories. Open http://localhost:9001/.
• The Accessibility panel of each Storybook story has 0 violations (aXe). Open http://localhost:9001/.
• Review tests are passing in the browser. Open http://localhost:8001/.
• Review markup conforms to SLDS by looking at DOM snapshot strings.
• Add year-first date and commit SHA to last-slds-markup-review in package.json and push.
• Request a review of the deployed Heroku app by the Salesforce UX Accessibility Team.
• Add year-first review date, and commit SHA, last-accessibility-review, to package.json and push.
• While the contributor's branch is checked out, run npm run local-update within locally cloned site repo to confirm the site will function correctly at the next release.

[futuremint]

Looks good!