Pasting into a participant field now preserves any text already there (Issue #2410) (#2635)

This modifies the behavious when pasting into the to, cc or bcc fields of a new draft.
Any text already in the input field is not automatically deleted. Rather the pasted
text is added to the already existing text. If the new contents of the email field now
matches as an email address (based on RegExpUtils.emailRegex().test()), then the address
is immediately tokenized. If the new input text doesn't match as an email, just append
the new text and wait for further input.

src/components/tokenizing-text-field.cjsx

@@ -528,8 +528,13 @@ class TokenizingTextField extends React.Component
 
   _onPaste: (event) =>
     data = event.clipboardData.getData('text/plain')
-    @_addInputValue(data)
-    event.preventDefault()
+    newInputValue = @state.inputValue + data
+    if RegExpUtils.emailRegex().test(newInputValue)
+      @_addInputValue(newInputValue, skipNameLookup: true)
+      event.preventDefault()
+    else
+      @_refreshCompletions(newInputValue)
+
 
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