Enhance commit to offer meta tx message

[zajck]

Closes #815

Currently, when a buyer is committing to an offer via metaTransaction, the only displayed value is offerId.

With this PR, the message to sigh is changed to contain other offer parameters:

• exchange token
• price
• seller deposit
• buyer cancellation penalty
• the start of the redemption period (in the format yyyy/mm/dd hh:mm:ss)
• dispute period duration (in days)
• resolution period duration (in days)

Note: changes apply only to metatransaction. Committing to the offer directly still requires only the offerId.

[coveralls]

coverage: 99.56% (+0.001%) from 99.559% when pulling fa2c48d on meta-tx-enhance-commit-to-offer-nicer into 9cc931b on main.