stream: Change button text to "Close" in copy email address modal.

This commit changes the button text from "Cancel" to "Close" in
copy email address modal, since it is weird to click on "Cancel"
button after having copied the email address and might confuse
users. Using "Close" makes it clear that the button is to just
close the modal.

web/src/stream_edit.js

@@ -513,6 +513,7 @@ export function initialize() {
             html_body: copy_email_address,
             id: "copy_email_address_modal",
             html_submit_button: $t_html({defaultMessage: "Copy address"}),
+            html_exit_button: $t_html({defaultMessage: "Close"}),
             help_link: "/help/message-a-stream-by-email#configuration-options",
             on_click() {},
             close_on_submit: false,