Stop crashing when the IA idea no longer exists.

I made a mistake when programming the migrated to thread redirect.
This rectifies it by moving the migrated check to after the exists /
ghosted check.

This also moves the thread -> idea migrated check to after the
permissions checks. There's no interface element exposed to migrate
non-general threads to IA ideas, but I think belts look smashing with
braces.
Merge pull request #310 from duckduckgo/non-existent-ia-ideas

Stop crashing when the IA idea no longer exists.