TextBlocks: render haml with layout

This commit changes the way haml TextBlocks are handled in two significant
ways:

1. we no longer allow Ruby code in TextBlock.body. We could add it back, but
quite frankly, I feel much better not having executable code in databases
and hence I would like to leave it that way for the time being.

2. The TextBlock body does is now embedded in a layout template, so it
should only really contain the actual content.

Templates have been updated accordingly, make sure you follow those changes
in your local database. Also, we need to make sure the TextBlocks on the
production system get updated at the same time this commit is deployed.