Continue the cleanup of the server code

* The Meetup API wrapper is now extracted in its own file (could
  probably be published as a separate Meteor package)
* Vote (and unvote) a topic now only requires to send the topic id
  (it used to send the whole document and to require at least the
  title, which was unnecessary and could have been used by malicious
  users)
* Remove useless `Meteor.startup` wrapper for the `Accounts`
  configuration.
* Remove useless server limitation on some collection hooks
* Reduce spaghetti code (some conditions where checked twice, and in
  method we can simply throw an error instead of wrapping the whole
  code in one more condition)
* Coding style: wrap files in 80 columns, test identity instead of
  weak equality, remove quotes in dictionaries keys, new line at the
  end of file, use `(err, res)` names for nodejs callbacks, etc.
* Add comments on top of files to explain what they do, and XXX
  comments for things to do.
* Improve the way we define authorizations by separating normal user
  and administrator in two distinct calls. (and add a few `fetch`
  arguments for efficiency)