Clean up specific invitation code mixed into general toolset

[asmecher]

The generic InvitationHandler class accidentally got some invitation-specific code mixed into it with #10576. The code specific to the user invitation should be moved to its respective handler, leaving InvitationHandler a generic tool for any kind of invitation.

[defstat]

@asmecher @withanage, @ipula has prepared the following PRs for this issue:

• main: refactor invitation page handler #11272

• stable_3_5_0: refactor invitation page handler #11266

These PRs complete the separation of generic and invitation-specific code relevant to this issue.
The respective ui-lib changes were merged yesterday separately.

@withanage @ipula @Tribunal33
There's a separate case in the "Edit User" flow — when a userId is provided that doesn't exist, it currently throws an HTTP 500 error. I believe that could be handled in a separate issue.

@asmecher
I don’t have the necessary permissions to fully merge the above PRs. Would you be able to assist with that?

[jardakotesovec]

Merged, sorry on jumping gun on the ui-library part.. I did not realise that is pkp-lib part while going through various PRs.. even though its quite obvious..

Tests are passing for me locally with these changes.. will watch out for that on the GA as well..

[ipula]

@asmecher @withanage, @ipula has prepared the following PRs for this issue:

* `main`: [refactor invitation page handler #11272](https://github.com/pkp/pkp-lib/pull/11272)

* `stable_3_5_0`: [refactor invitation page handler #11266](https://github.com/pkp/pkp-lib/pull/11266)

These PRs complete the separation of generic and invitation-specific code relevant to this issue. The respective ui-lib changes were merged yesterday separately.

@withanage @ipula @Tribunal33 There's a separate case in the "Edit User" flow — when a userId is provided that doesn't exist, it currently throws an HTTP 500 error. I believe that could be handled in a separate issue.

@asmecher I don’t have the necessary permissions to fully merge the above PRs. Would you be able to assist with that?

I create a PR for that here