feat(app-shell): to add label to maintenance page layouts

[tdeekens]

Summary

This pull request adds a label which is used on the images alt to all maintenance pages.

Description

Our maintenance pages mostly have: title, two paragraphs and an image. The image itself however doesn't contain any alt text. It would be good to mostly use the title as the alt text on the image.

This also eases querying for the image with byRole('img', { name: //i }) under test.

I had multiple solution in mind when working on this:

1. Add a optional string of label which goes to the img alt. The img alt needs to be a string and can't be React.Node
2. Just use the title and pass it as the alt. Slighly less flexible but less noisy for consumers. However, the title is an React.Node as it's a FormattedMessage while the alt needs a string.
3. Refactor everything to just accept *IntlMessage for instance titleIntlMessage. This however is a breaking change to the public component.
4. Offer two sets of props: a bit too much work for a little thing like this

[changeset-bot]

🦋 Changeset is good to go

Latest commit: 32aed78

We got this.

This PR includes changesets to release 6 packages

Name	Type
@commercetools-frontend/application-components	Minor
@commercetools-frontend/application-shell	Minor
merchant-center-application-template-starter	Patch
playground	Patch
@commercetools-local/visual-testing-app	Patch
@commercetools-website/components-playground	Patch

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[ahmehri]

Thanks!