Convert "organization membership type" field to many-to-many.

The "organization membership type" field is currently a one-to-many relationship, but @jvallera wants to be able to use it to classify orgs as both "founding members" and "highly active members", and possibly other things later on.

This is a simple migration that consists of 3 mini-migrations:

* A schema migration adding the `membership_types` `ManyToMany` field to the `Organization` model.
* A data migration copying the `membership_type` `ForeignKey` field to the aforementioned `ManyToMany` field on all organizations.
* A schema migration removing the aforementioned `ForeignKey` field.

We also change the display of the membership type to use the bootstrap "label" style for each type, like so:

![capture](https://cloud.githubusercontent.com/assets/124687/4240848/c48326b0-39ee-11e4-980c-d254a00a2d45.PNG)

In the future, such labels will be links that take the user to a page listing all organizations of the typed that was clicked on. (This feature is out of scope for this PR, though.)