[#121] Add unique constraint to Organisation.iati_org_id

Add migration for the constraint

Bugfix in BudgetItem.get_label()
[#121] Fix borked migrations

Replace migration 0032 to change empty string values in
Organisation.iati_org_id to NULL values to prepare for the unique
constraint on the field.

Add schema migration 0033 to set the unique constraint on
Organisation.iati_org_id and to create the InternalOrganisationID model.

Add data migration 0034 to add custom data to Cordaid and DGIS
organisation objects
Fix for #199 increased BenchmarkName limit from 50 to 80
Actually added migration from previous commit.
[#207] Add notes field to PartnerSite model

Add PartnerSite.notes field to models.py

Add PartnerSiteAdmin.restricted_fieldsets leaving out the notes field
and add methods to PartnerSiteAdmin to choose which fieldsets to use
based on user permissions for the PartnerSite model.

Add PartnerSiteAdmin.get_list_display() to only show the notes field in
the list view for users with privileges as above.

Add migration for the PartnerSite.notes field.
[#207] Add 'enabled' column to the admin list view for ParterSite
[#207] Code comment fix
[#203] Add PartnerType model

The PartnerType is a helper model containing one object for each partner
type organisations can assume in a Partnership.

Add the model definition to rsr.models.

Add Organisation.partner_types many-to-many field that enumerates the
types of partner a certain organisation can assume.

Add schema migration 0037 for PartnerType.

Add data migration 0038 that populates Organisation.partner_types based
on the current partnerships for each organisation.
[#203] Add object creation for PartnerType

Add signal to rsr.management that adds the objects for each partner
type to PartnerType if they are not already present
[#203] Add Organisation.partner_types to admin

Set Organisation.partner_types to be read only unless you have change
permission on Organisation.

Change the help text for Organisation.partner_types field in the admin
depending on the read only status.
[#203] Add RSR_PartnershipInlineForm and
RSR_PartnershipInlineForm.clean_partner_type()

RSR_PartnershipInlineForm.clean_partner_type() checks if the
organisation is allowed to assume the given partner type based on the
objects in Organisation.partner_types for the org in question.

Small refactorings in RSR_PartnershipInlineFormFormSet.
Make the admin form for Organisations a little more robust

Check for the existance of the inline formsets before trying to access
them.
[#203] Add column lising allowed partner types to Organisation list

The list is only visible if you have change permission on Organisation,
currently meaning Akvo staff groups only
Merge branch 'feature/limited_partner_types' into develop