Create a "locked" property on a claim

[cguess]

If we have multiple people categorizing claims at the same time we dont' want overlap. To do so we'll add a "locked_at" property to the database. This will be a timestamp so we can can just look for claims where "locked_at" is nil or older than a certain time. On save this will clear.

Probably have a job to clear the locked_at as well after a certain amount of time.