After turning off iteration_cadences FF,
creating a new iteration should be possible.

1. The mutation resolver for creating an iteration
is updated to accept a request even if there are
mulitple cadences when iteration_cadeces FF is off.

2. Iteration model is updated so that the first cadence
is used as the default cadence for each group when
creating a new iteration

EE: true
Changelog: fixed

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Fix indentation on set_cadence method