Implement the topics

[Francisco]

Replace meetings with topics of kind meeting.

Implement new schema:

{
  kind: String, // IDEA, INFO, TODO, DECISION or MEETING
  name: String, // title of the topic
  text: String, // can be markdown
  author: String, // maybe should be `member` instead

  targets: [String], // people assigned to this topic or people who were present if type==MEETING

  closed: {type:Boolean, default: false},

  // only used on if type==DECISION
  poll: {
    kind: {type: String, default: 'text'}, // TEXT or IMAGE
    options: [{
      content: String,
      votes: [String]
    }]
  },
  duedate: {type: Date},

  tags: [String],

  posted: {type: Date},
  updated: {type: Date},
}