SOC-4317: [IE8] Javascript error when deleting letters in Activity Composer


Problem analysis:
* When deleting characters in Activity Composer, triggerOninputMention is invoked to update the text content.
  The property "textContent" isn't supported on IE7 or IE8 browser.

Fix description:
* Only update the text content when currentSelection is valid.
  In case of IE7 and IE8, use "innerText" to get text content.