Display batch statuses in Mail Status Admin UI

Right now we display statuses for each mail. We need to also display info about the batches.

The proposal is to:

• Introduce a new DB table for batch data:
  • batch id
  • batch status: in progress, stopped, finished
  • number of mails in the batch
• When click on Mail Status in Admin UI, display a LT displaying batch statuses with following columns:
  • batch id
  • batch status: in progress, stopped, finished
  • number of mails in the batch
  • number of mails that have been sent successfully
  • number of mails that have failed to be sent
  • actions column with buttons: "Details", "Delete", "Resend"
• When clicking on "Details", display the LT with one status line per mail for the batch (the LT that is currently displayed)
• When clicking on "Resend", resend all mails that have not been sent successfully (on error or in the prepare state). The "Resend" button should be greyed out till the batch status is "finished". See XWIKI-11652
• When clicking on "Delete", ask for confirmation and delete all mail statuses for the batch + the batch status itself. See XWIKI-11653