GET /api/user/participants has not the expected behavior when using multiple parameters.

[doumdi]

Describe the bug
Some parameters are not taken into consideration when querying participants due to if/elif structure of the implementation of the /api/user/participants endpoint.

To Reproduce
Ex: Query a participant with username and specify its id_site and id_project. Will return participants with the name in any project or site.

Expected behavior
Handle multiple parameters and filter results accordingly.