Use Chargeable field for determining measure in rate editor

[lpichler]

previously ChargebackRate(variable detail in code) model had relation to ChargebackRateDetailMeasure model. But we removed it and we relations are:
ChargebackRate -> ChargeableField -> ChargebackRateDetailMeasure

So we need to get ChargebackRateDetailMeasure from ChargeableField and it was causing that select box was disappeared.

before

after

Testing steps

1. CI -> Chargeback -> Rates
2. Add and edit existing rate
3. See it there are checkboxes in per unit column.

@miq-bot assign @mzazrivec
@miq-bot add_label bug

[miq-bot]

Checked commit lpichler@9e967cd with ruby 2.3.3, rubocop 0.47.1, and haml-lint 0.20.0
1 file checked, 0 offenses detected
Everything looks fine. 🏆