fix: perun webflow integration

Description

The place of integration of our perun flow to CAS changed to right after SERVICE_CHECK. For more details check the docs added in this MR too. Attribute mappings and some other stuff can be different for each service(SP) and that wasn't taken into acount before. This should fix it.

How to test

See Testing here.

Author's checklist

• I have followed the contribution guidelines
• This MR has been tested or does not change functionality
• I have added relevant merge request dependencies (if this MR has any)
• I have added the correct labels
• I have assigned reviewers (if any are relevant)
• I have edited the documentation (if the changes require it) or I have noted the need for the change if I do not have access to the documentation
• I have marked all introduced BREAKING CHANGES or necessary DEPLOYMENT NOTES in the commit message(s)

Reviewer's checklist

• This MR has been tested or does not change functionality
• This MR has correct commit message format

Other information

When logging directly to IDP(through CAS login) our flow didn't trigger as expected and GeneralSuccesPage for login was displayed. When logging from service it correctly triggered.

EDIT: Just found out that logging directly to CAS can be disabled. More here. If we do disable it, we can asure that our flow will always run. However our flow serves mostly for service integration (AFAIK) so it may not be an issue.

Or we can just change the generic login page to include list/search box of services but that would require more work.

UPDATE: Our flow now runs in the both type of logins - sp and idp initiated.

Related issues

closes STR-1586