#77 Add a user modified flag.

Change the transcription table schema.

When we update a transcription generator tool to fix things, we need to
regenerate the transcriptions. But we don’t want to mess with users by
regenerating transcriptions that has been manually modified to fix things
that may not be fixed by the generator yet. So we’ll flag manually modified
transcriptions as such, and only update the non manually modified ones.
 #77 Store sentence script.

Added a field in the `sentences` table to store the script. This is only used
for languages that can be written using more than one script, as defined in
the Transcription model.

Updated the transcriptableToWhat() function to make use of this.
 #77 Validate against possible transcriptions.

Added validation rules to prevent invalid transcriptions, giving the source
language, the source script and the target script.
 #77 Function to call transcription generator.
 #77 Support chained transcription.
 #77 Fixed unicity validation.
 #77 Function to bulk add every transcriptions.

And refactored mock generation in the tests.
 #77 Add transcriptions on sentence creation.