Improve MINEVA-NET basic functionality

Some remarks after working with MINERVA-NET:

• In the project view: A column with radio buttons (local/share) should be replaced with tickboxes (text: share)
• When clicking on "Register in MINERVA-NET" the "Project URL" should not be editable, as it is anyway taken from the Configuration > Server > MINERVA Root URL, if this value is missing, display a warning message: "Cannot join MINERVA-NET, please provide 'MINERVA Root URL' in 'Configuration > Server'"
• "Value" column should be wider, at least 3x, so URLs and emails are fully visible
• When sharing a private project, the error message should say: "This project is not publicly visible. To share, provide view access to the user 'anonymous'"