Allow handler to accept invitation to application

[pikariop]

Fix: #3100

Allow a handler to join an application as a member, and be added as a member. If desired, the owner can toggle back the old behaviour from a given workflow's settings ("Disable actions").

Checklist for author

Reviewability

• Link to issue

Documentation

• Update changelog if necessary
• Update docs/ (if applicable)

Testing

• Complex logic is unit tested
• Valuable features are integration / browser / acceptance tested automatically