Support text orientation on transcription

Add a new enum field named orientation on Transcription, non nullable, with these possible values:

• horizontal-lr (default value)
• horizontal-rl
• vertical-rl
• vertical-lr

This loosely match the CSS writing-mode attribute, where :

• horizontal-lr is writing-mode: horizontal-tb;
• horizontal-rl is writing-mode: horizontal-tb; text-align: right; (for arabic)

This value must be editable in the admin interface.

There is no modification on any endpoint here