Warning on Dispute button

[jjeffryes]

It's not clear to the user what opening a dispute will do, nor that it's a one-way process that can't be undone.

When the open dispute checkbox is clicked, show a "Warning, this will [explaination of dispute process]" message.

Also change the "100% of funds will go back to the buyer" message to subtract the moderator's fee.

[jjeffryes]

Note: we should consider making this part of a larger refactoring of the dispute button design, some users find the current checkbox design confusing.