fix(DraftJSMentionSelectorCore): Dynamically removed and reapplied alert Mention feature

[fsuarez-globant]

In order to make multiple announcements using the same alert element, the role attribute must be dynamically removed and reapplied every time an alert is made. I used of ARIA alerts to inform screen reader users of how many results have been found (e.g., "3 users found", "No users found", etc.)

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