Add Welcome Mat Component

[aswinshenoy]

Fixes #1876

Additional description

Adds Welcome Mat component following the SLDS pattern

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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.

Required only if there are markup / UX changes

• 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.

[aswinshenoy]

This is just an initial commit, and these are the shortcomings as of now-

• DSR Modal doesn't support slds-modal_small, so the welcome mat cannot be correctly fit like in SLDS. I have not worked on closing and opening of modal, as I am not sure whether the component will be manually called under a or be always under a modal.
• Trailblazer badges

The code is also pretty rough right now, I will be working on this in the coming days :)

[interactivellama]

I will try to look at this next week.

[kevinparkerson]

@aswinshenoy please add ids to every component that accepts them in your examples.

Also add the welcome-mat docs.json contents to the components array in the LDSR's package.json file

[kevinparkerson]

@aswinshenoy looking good so far! I'll take another look once you've addressed these issues and updated your browser tests

[aswinshenoy]

@kevinparkerson I made the suggested changes and wrote browser tests. I missed writing a test for checking whether clicking on Tile takes to the href link, would that be necessary? That is perhaps the only interactivity test I guess we will have, the others I have written are purely markup checks (something snapshot testing would have done).

Please Review :)

[kevinparkerson]

Checked it over, it's good to go! Thanks for all your good work on this @aswinshenoy!!!