comments: Add temporary edits setting.

[lukebp]

Allowing temporary comment edits is desirable for certain applications and should be built into the comments plugin.

[amass01]

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

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