update the structure of the mri objects in the DB to deal with volume and text annotations in the same way

Created by: r03ert0

The mri object in the DB looks currently like this:

mri: {
  atlas: [
    { owner:, created:, modified:, access:, type:, filename:, labels:, project:, name:}
  ],
  annotations: {
    thing1:,
    thing2:,
    ...
  }
}

atlas means in fact annotations of type volume, and annotations means annotations of type text (and in the future there may be more type of annotations added).

The mri structure should be the same for any type of annotation. Like this:

mri: {
  annotations: [
    { owner:, created:, modified:, access:, type:, filename:, values:, project:, name:}
  ]
}

(the labels field will then just be replaced by the values field when type is volume)