Handle merging blocks a bit better

The following logic is used when picking which block
is used (in order):

• Which one expires later
• If it blocks account creation
• If it blocks email
• If it blocks editing your own user talk page
• The block against the user we're merging into

Bug: 39470
Change-Id: I86c3cfe7565eeddbaf9dabe8bab6e21899e30e77