Notify when account changes instead of when deleted

Previously, the notification was posted when the account was preparing
to be deleted. Besides not being accurate, this could be called multiple
times, once per managed context.
Reload table when blog list changes

The blog list will not change often, and trying to manage rows/sections
individually is tricky, specially when there’s not a 1-1 match to the
data source (“Add site” row).

Reloading the whole table when the results change is less prone to
errors and shouldn’t affect performance much, since it doesn’t happen
often.
Only consider self hosted for the welcome screen

We are already checking for an account to see if we are logged into
WordPress.com.

The reason for this is that now the account changed notification is
posted as soon as the default account is set to nil, but at that point
the account and its blogs still exist.

The `noBlogsAndNoWordPressDotComAccount` method would always return `NO`
on sign out, since deletion is done asynchronously.
Refactor blog counting methods
Merge pull request #1123 from wordpress-mobile/issue/1112-blog-list-corrupted-sign-out

#1112 Blog list corrupted sign out