Check recipe flags at schedule creation (including fix for youzim.it)

[benoit74]

Rationale

Fix #864 (again)
Fix #867 (mandatory to deploy #864 on youzim.it)

Changes

• new environment variable ZIMIT_DISABLE_ZIM_FILENAME_CHECK ZIMIT_USE_RELAXED_SCHEMA which must be set to disable the ZIM filename check
• new logic to use this variable to select a relaxed offliner schema definition
• previous changes from Check schedule flags validity also when creating a schedule from scratch #865

[benoit74]

Nota: do not merge until decision has been taken on openzim/zimit-frontend#42 + synchronization with Travis is needed

[rgaudin]

Thank you ; please take a look at suggestions and question

[benoit74]

Following your review, I made following changes:

• create a ZimitFlagsSchemaRelaxed class which override the zim_file field to remove the validation
• use conditionally the ZimitFlagsSchemaRelaxed in get_offliner_schema based on environment variable
• renamed the environment variable to ZIMIT_USE_RELAXED_SCHEMA to better match its "new" purpose + transformed into a bool
• fixed tests so that all cases are now covered by manipulating the constant (manipulating the constant is where my bad Python programming made me fail previously)

[rgaudin]

LGTM ; couple minor suggestions. I'll let you decide what to do. Please merge yourself

[benoit74]

Suggested changes made, makes a lot of sense.

I converted this PR to Draft so that it is not merged by accident, we need to synchronize with Travis + prepare new env variable for youzim.it