comments: Add temporary edits setting. about politeia HOT 1 CLOSED

Giving the fact that politeiad comments plugin already has an
edit command, here are the TODOs needed to have it fully implemented
and functioning:

• Add two new settings to the comments plugin:

1. allowedits - global flag to enable comments edits.
2. editperiodmax - defines the maximum period of time in seconds where comments are still editable.

• Add logic to politeiad to ensure edits are allowed only during the time interval set
  by the new editperiodmax setting.
• Add the new settings to the comments API policy route reply.
• Add an /edit route to the comment API.
• Add a pictl commentedit command.

GUI:

• Fetch new editperiodmax setting from the comments API policy.
• Add an edit icon for comment author while comment is still editable.
• Use the new /edit route to post edits.

from politeia.