Fixes #1315: Adds a small section on the homepage with connected projects and update the authenticate function

[devhashtag]

The authenticate function now only checks if a project is in a session when it is accessed using a GET request (i.e. the user did not send a private code). This prevents the user from opening a project with the wrong private code. Also, the home page has a section that lists the connected projects (if there are any) and provides links to their respective pages.

The following tests are added:

• test_project_list_redirection tests that connected projects are displayed on the home page.
• test_get_auth tests that projects in the current session are accessible using the project link, without having to provide a password.
• test_post_auth_wrong_password tests that wrong private codes are rejected for connected projects.
• test_post_auth_correct_password tests that correct private codes are accepted for connected projects.