Change data type of columns in many relations

As a result of the discussion in issue #715 I created a set of commits that introduces new data types for many columns of the database. We hope to improve performance a bit, which was in tests the case for at least some queries.

Also, the size of the tables went down a bit. This is the size of the 20 biggest relations *before* the migrations of this pull request:

Relations, indices and etc. separate:

               relation               |  size
--------------------------------------|---------
 public.treenode                      | 1205 MB
 public.treenode_location_z_index     | 163 MB
 public.treenode_location_x_index     | 163 MB
 public.skeleton_id_treenode_index    | 163 MB
 public.treenode_parent_id            | 163 MB
 public.treenode_pkey                 | 163 MB
 public.treenode_id_key               | 163 MB
 public.treenode_location_y_index     | 163 MB
 public.log                           | 119 MB
 public.review                        | 98 MB
 public.class_instance                | 81 MB
 public.treenode_connector            | 74 MB
 public.class_instance_class_instance | 68 MB
 public.treenode_class_instance       | 61 MB
 public.review_skeleton_id            | 37 MB
 public.review_reviewer_id            | 37 MB
 public.review_project_id             | 37 MB
 public.review_pkey                   | 37 MB
 public.review_treenode_id            | 37 MB
 public.connector                     | 20 MB


All data belonging to a certain relation aggregated:


               relation               | total_size
--------------------------------------|------------
 public.treenode                      | 2346 MB
 public.review                        | 281 MB
 public.class_instance_class_instance | 231 MB
 public.treenode_connector            | 210 MB
 public.class_instance                | 193 MB
 public.treenode_class_instance       | 165 MB
 public.log                           | 135 MB
 public.connector                     | 37 MB
 public.change_request                | 4304 kB
 public.django_session                | 2744 kB
 public.connector_class_instance      | 136 kB
 public.message                       | 120 kB
 public.auth_permission               | 112 kB
 public.django_admin_log              | 104 kB
 public.guardian_userobjectpermission | 88 kB
 public.class                         | 80 kB
 public.djcelery_taskstate            | 80 kB
 public.auth_user                     | 72 kB
 public.auth_user_groups              | 72 kB
 public.auth_user_user_permissions    | 72 kB

And this is the data *after* the migrations have been applied:

Relations, indices and etc. separate:

               relation               |  size
--------------------------------------|---------
 public.treenode                      | 1020 MB
 public.treenode_parent_id            | 163 MB
 public.treenode_location_y_index     | 163 MB
 public.treenode_location_x_index     | 163 MB
 public.treenode_id_key               | 163 MB
 public.treenode_location_z_index     | 163 MB
 public.skeleton_id_treenode_index    | 163 MB
 public.treenode_pkey                 | 163 MB
 public.review                        | 111 MB
 public.log                           | 104 MB
 public.class_instance                | 67 MB
 public.treenode_connector            | 57 MB
 public.treenode_class_instance       | 50 MB
 public.class_instance_class_instance | 50 MB
 public.review_skeleton_id            | 37 MB
 public.review_reviewer_id            | 37 MB
 public.review_project_id             | 37 MB
 public.review_pkey                   | 37 MB
 public.review_treenode_id            | 37 MB
 public.class_instance_user_id        | 19 MB

All data belonging to a certain relation aggregated:

               relation               | total_size
--------------------------------------|------------
 public.treenode                      | 2161 MB
 public.review                        | 294 MB
 public.class_instance_class_instance | 213 MB
 public.treenode_connector            | 193 MB
 public.class_instance                | 179 MB
 public.treenode_class_instance       | 154 MB
 public.log                           | 119 MB
 public.connector                     | 35 MB
 public.change_request                | 4040 kB
 public.django_session                | 2744 kB
 public.connector_class_instance      | 128 kB
 public.message                       | 120 kB
 public.auth_permission               | 112 kB
 public.django_admin_log              | 104 kB
 public.guardian_userobjectpermission | 88 kB
 public.djcelery_taskstate            | 80 kB
 public.class                         | 80 kB
 public.auth_user_groups              | 72 kB
 public.auth_user                     | 72 kB
 public.auth_user_user_permissions    | 72 kB