Schedule configuration flags must also be validated at schedule creation

[benoit74]

• Location: API

Problem

Schedule configuration flags have various constraints which must be checked. These constraints are check when the schedule configuration is updated (PATCH /schedule/{schedule_id}) but not when the schedule is created from scratch (POST /schedule).

This has almost zero impact on farm.openzim.org since the UI does not allows to create schedules from scratch but it has a very significant one on youzim.it where the UI is always creating schedules from scratch (and most probably, same issue impact WP1, but I'm less familiar with it).

Once fixed, this will solve (first part of) openzim/zimit-frontend#40

Reproducing steps

Create (not clone and patch) a new schedule with a very long description (longer than 80 chars). It will succeed while it should fails.

[benoit74]

Change reverted due to openzim/zimit-frontend#43 (and probably other impacts on WP1)