Builds : add build hash and impact on API

[pierrotsmnrd]

As agreed in issue #311 the preferred way to represent a build is :

May 31, 2022 - 17:00 (a1b2c3)

i.e. "date of build" and a hash.

This implies multiple impacts and choices :

• First, add a hash field to build. This will require using Alembic migrations to update the model and the DB.
• Impact the API so the hash is returned under the /api/v1/build/ and /api/v1/build/<build_id> endpoints.

On the UI, I suggest to use the field scheduled_on as date of build, to ensure all the builds will have a date.