[ENH] Newsletters: avoid Apache and/or cgi and/or browser timeouts when sending a lot of emails by sending info to the browser for each user (using php flush functions).
Now, when sending a newsletter, a page titled "Sending Newsletter ..." is displayed with an iframe that shows a realtime list of the emails which have already been handled by the sending process. For each mail, it also indicates if it was OK or if it failed.
The iframe scrolls down automatically and includes a summary at the end when the job is finished.
If the temporary directory of Tiki is a subfolder of Tiki installation (like the default one "temp/"), a link to download the log (list of emails and status) is also generated.
Refreshing the page (e.g. by pressing 'F5') should not anymore send the newsletter twice without prompting for the confirmation message.
+ Removing the "sender_email" variable which is not needed (the sender_email pref is already used by default in tikimaillib when nothing is specified)