Improved user management

[anthonygego]

• Page to manage users manually
• Initialization : superadmin should be present already present in DB

[shlomihod]

A manual option for populating the DB with CSV files with all the users' data is with mongoimport: mongoimport -d INGInious -c users --type CSV --file ./users.csv --headerline

while the CSV file has the headline: email,realname,password,username