Improve banned and deleted user handling on Transaction Page

[SariSaar]

This change improves banned and deleted user handling on the transaction page:

• For inquiry and negotiation processes, the initial message is saved in the transaction's protected data instead of sent as a message. Now, when the sender is banned, those messages are replaced with the same default marketplace texts as regular messages.
• Transaction process state specific extra messages are hidden for banned and deleted users.
• Transaction page action buttons are disabled when the counterparty is banned or deleted, because transactions cannot move forward in that circumstance.