closes https://github.com/TryGhost/Team/issues/2380
- improved offer validation for `amount` field to cover all type/amount cases
- added validate-on-blur to the amount field to match our standard validation behaviour
- added re-validation of the amount field when the type is changed and the amount gets reset
- removed the internal parsing of a decimal trial days entry to an integer so the field value matches what is set internally and we let the user know that partial trial days are not supported
Non-user-facing refactors:
- renamed `_saveOfferProperty` to `_updateOfferProperty` to better reflect what it does
- fixed missing indentation for conditional blocks in the offer template
refs https://github.com/TryGhost/Team/issues/1236
Stripe allows coupon names to be only upto 40 chars long, while Ghost allowed them to be 191 chars. This change updates the admin validation to restrict name to 40 chars to match Stripe limit
