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