Ensure alert popups retain "X" to close, but with screen reader labels

[lorawoodford]

#1762 accidentally removed the "X" that allows users to close a number of ArchivesSpace alerts by placing the inside of a screen reader only class. This removes that addition and uses javascript to add an aria-label to the link for accessibility purposes.

Description

Related JIRA Ticket or GitHub Issue

How Has This Been Tested?

Confirmed visually that the X returned, utilized ChromeVox and Wave plugin to confirm that the aria-label is being presented to screen reader users. Broken Selenium tests that alerted me to this regression are now passing.

Screenshots (if appropriate):

X's missing in 2.7.1:

X to close back on alerts:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have read the CONTRIBUTING document.
• I have authority to submit this code.
• I have added tests to cover my changes.
• All new and existing tests passed.