Allow vote creator to add a comment to a proposed unbond transaction

[chrishobcroft]

Current situation

When an unbond is proposed for a vote, the description for the vote is something like:

Livepeer Delegator: Unbond 20.246720785279819288 LPT tokens

This provides the voter with information about what the unbond amount is, but nothing about why the unbond is being performed.

Proposed solution

To add an optional "Reason" field to every vote proposed, to allow a vote-proposer to add a text comment to the transaction.

This could be included in the vote like:

Livepeer Delegator: Unbond 20.246720785279819288 LPT tokens. Reason: to withdraw then create gitcoin bounty to fund change request defined in issue 5.