Fix tags select clear button

[Oaphi]

closes #1462

This PR consists of a couple of fixes for tag handling:

• save_draft method on the posts_controller now validates that the incoming tags array does not consist of empty elements only;
• save_draft method removes existing tags array before inserting new elements (ensures correct partial updates);
• reduced delay before a draft is saved to 1 second (3 seconds is way too long to be useful);
• tag clear button requires placeholder to be set to work (select2 doesn't check if placeholder is provided before accessing its id property);
• include_blank option should not be used with multiple select fields. Otherwise, select2 will add an ephemeral tag (with an empty name) every time it's cleared;