turn MessagePage migration into an upgrade task

It was not really migrating anything - only converting data.

Having it as a migration caused all kinds of trouble with newer data models
cleanup keys table from translations engine

otherwise we can't create a new keys table
convert_message_pages needs the environment
raise proper error if thumbnail definition is missing
enable odt thumbnails of word documents

They might be handy anyway but in particular we still have some of these on the live servers and the assets can't be removed if the thumbdef is not around because proxy? will err out.

see https://labs.riseup.net/code/issues/4336

Conflicts:
	test/unit/asset_test.rb
revert deploy branch - was changed for staging

This was never meant to be commited. Only used for staging the migration fixes.
Merge pull request #44 from azul/bugfix/fix-migrations

Bugfix/fix migrations