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.
Sometime the first call to SQL::Translator::translate crashes.

This adds calls for the new and old schemas and discards the result as a
workaround.

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Merge pull request #310 from duckduckgo/non-existent-ia-ideas

Stop crashing when the IA idea no longer exists.
Merge pull request #311 from duckduckgo/db-upgrade-workaround

Sometime the first call to SQL::Translator::translate crashes.