Add Cancel button to the Edit description form

[mawinter69]

When one clicks on Add or Edit description, it is not possible to cancel this. The only action to revert from here is to reload the page. This is bad UX.

This adds a Cancel button to overcome this.

The buttons are now aligned to the left.

Before:

After:

See JENKINS-XXXXX.

Testing done

• create freestyle and job with description and saved it, clicked on "Edit description" (it is in the app-bar), changed text, then pressed "Cancel" -> original text is shown, clicked again on "Edit description" -> form is loaded properly
• Done the same for a matrix job (the button is next to the form)
• And checked this for the view description

Proposed changelog entries

• Add Cancel button to the Edit description form

Proposed changelog category

/label rfe,web-ui

Proposed upgrade guidelines

N/A

Submitter checklist

• The Jira issue, if it exists, is well-described.
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Desired reviewers

@mention

Before the changes are marked as ready-for-merge:

Maintainer checklist

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
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