Implement basic dice system

[danneu]

Schema changes:

-- TODO: campaigns.topic_id NULL
CREATE TABLE campaigns (
  id             serial           PRIMARY KEY,
  user_id        int              NOT NULL REFERENCES users(id),
  title          text             NOT NULL,
  markup         text             NULL,
  html           text             NULL,
  created_at     timestamptz      NOT NULL DEFAULT NOW(),
  roll_count     int              NOT NULL DEFAULT 0,
  last_roll_at   timestamptz      NULL
  --, last_roll_id   int              NULL REFERENCES rolls(id)
);

CREATE TABLE rolls (
  id             serial           PRIMARY KEY,
  user_id        int              NOT NULL REFERENCES users(id),
  campaign_id    int              NOT NULL REFERENCES campaigns(id),
  syntax         text             NOT NULL,
  rolls          json             NOT NULL,
  total          int              NOT NULL,
  note           text             NULL,
  created_at     timestamptz      NOT NULL DEFAULT NOW()
);

-- TWEAKS

ALTER TABLE campaigns ADD COLUMN last_roll_id int NULL REFERENCES rolls(id);

-- INDEXES

-- Looking up all campaigns started by a user
CREATE INDEX campaigns__user_id ON campaigns (user_id);
-- Sorting campaigns by latest roll
CREATE INDEX campaigns__last_roll_id ON campaigns (last_roll_id);
-- Looking up all rows for a campaign
CREATE INDEX rolls__campaign_id ON rolls (campaign_id);