add new invite/<token_type>/check/<token> endpoint

having `/invite/service/<token>` and `/invite/service/<id>` as two
separate routes (the first to validate an invite token, the second to
retrieve invite metadata) technically works. Routes are matched from
first to last until a match is found. The metadata endpoint only accepts
UUIDs, so requests with a UUID will be picked up by the correct
endpoint, while requests that don't look like a UUID will carry on
searching for an endpoint, and will find the token validation endpoint.

So while this works correctly for our normal expected input, it only
does so _because the UUID endpoint is first in the file_. This isn't
great, and it makes it harder to reason about the URLs when looking at
them.

To solve this, create the new `invite/service/check/<token>` endpoint.
For backwards compatibility, assign this in parallel with the existing
route - once the admin uses the new route we can remove the old route
and make better guarantees about what endpoint is being hit.

app/organisation/invite_rest.py

@@ -122,6 +122,7 @@ def get_invited_org_user(invited_org_user_id):
 
 
 @organisation_invite_blueprint.route('/invite/organisation/<token>', methods=['GET'])
+@organisation_invite_blueprint.route('/invite/organisation/check/<token>', methods=['GET'])
 def validate_invitation_token(token):
 
     max_age_seconds = 60 * 60 * 24 * current_app.config['INVITATION_EXPIRATION_DAYS']

app/service_invite/rest.py

@@ -100,6 +100,7 @@ def get_invited_user(invited_user_id):
 
 
 @service_invite.route('/invite/service/<token>', methods=['GET'])
+@service_invite.route('/invite/service/check/<token>', methods=['GET'])
 def validate_service_invitation_token(token):
 
     max_age_seconds = 60 * 60 * 24 * current_app.config['INVITATION_EXPIRATION_DAYS']