Allow user to initiate arbitration once locktime has expired

[ghost]

Problem description:

Normal sequence of events is user opens mediation, mediation completes and then user has option to open arbitration once locktime has expired. In the unusual cases, users might not be able to open mediation, or mediation might not close (as described in #4101). When that happens, arbitration cannot be initiated.

Ruleset for opening mediation or arbitration

CTRL-O opens a mediation ticket when either:

• trade is not in dispute,
• mediation has been initiated and the locktime has not expired.

else CTRL-O opens an arbitration ticket when either:

• trade mediation has closed,
• mediation result was rejected,
• locktime has expired.

To accomodate the fact that users may need to open arbitration manually, this change allows it upon locktime expiry.

Closes #4101

[ripcurlx]

utACK