rename invite blueprints

nb: the routes are not changing as part of this, only file paths and
blueprint names.

invite -> service_invite

this blueprint handles fetching invites for a service, creating invites,
etc.

accept_invite -> global_invite

this blueprint handles accepting invites for now, but will also involve
retrieving service/org user invite data without knowing the service/org
id associated. i'm not in love with this name and open to suggestions,
but i wanted to contrast it from service_invite and
organisation/invite_rest.py.

app/global_invite/rest.py

@@ -0,0 +1,40 @@
+from flask import Blueprint, current_app, jsonify
+from itsdangerous import BadData, SignatureExpired
+from notifications_utils.url_safe_token import check_token
+
+from app.dao.invited_user_dao import get_invited_user_by_id
+from app.dao.organisation_dao import dao_get_invited_organisation_user
+from app.errors import InvalidRequest, register_errors
+from app.schemas import invited_user_schema
+
+global_invite_blueprint = Blueprint('global_invite', __name__)
+register_errors(global_invite_blueprint)
+
+
+@global_invite_blueprint.route('/<invitation_type>/<token>', methods=['GET'])
+def validate_invitation_token(invitation_type, token):
+
+    max_age_seconds = 60 * 60 * 24 * current_app.config['INVITATION_EXPIRATION_DAYS']
+
+    try:
+        invited_user_id = check_token(token,
+                                      current_app.config['SECRET_KEY'],
+                                      current_app.config['DANGEROUS_SALT'],
+                                      max_age_seconds)
+    except SignatureExpired:
+        errors = {'invitation':
+                  'Your invitation to GOV.UK Notify has expired. '
+                  'Please ask the person that invited you to send you another one'}
+        raise InvalidRequest(errors, status_code=400)
+    except BadData:
+        errors = {'invitation': 'Something’s wrong with this link. Make sure you’ve copied the whole thing.'}
+        raise InvalidRequest(errors, status_code=400)
+
+    if invitation_type == 'service':
+        invited_user = get_invited_user_by_id(invited_user_id)
+        return jsonify(data=invited_user_schema.dump(invited_user).data), 200
+    elif invitation_type == 'organisation':
+        invited_user = dao_get_invited_organisation_user(invited_user_id)
+        return jsonify(data=invited_user.serialize()), 200
+    else:
+        raise InvalidRequest("Unrecognised invitation type: {}".format(invitation_type))