Newsletter create/edit interface

[ketan404]

Fixes to be done -

• Campaigns - Change to singular i.e. Campaign. There's going to be one and only one Campaign associated with a newsletter.
• Only two status values should be allowed to be selected - Draft and New. Other two values - related to Queing and sending should be put by the respective commands.
• Change the tags from type checkboxes to Multiple select menu. This menu will have a fixed height and will not affect the height of the page.