Move .htpasswd to studenti

Because I would like to use a private git repository to keep
track of the data and because I would like to track the passwords
as well, I am moving the password file to studenti.

The mechanism to track the content of the studenti directory
using git will come next.
mailer.js testability improvements

1) Wrap loading settings from disks into an init() callback to
   avoid race conditions and enable testability.

2) Separate the function that really sends the email to make it
   possible to invoke it from outside, for testability.

3) Propagate changes (namely #2) to frontend.js.
mailer: customize the sender via config file
add init_students.js: inits students database

Also:

a) Remove csvtojson.js, which was merge into init_students.js.

b) Change the default students database name to `studenti/iscritti.json',
   to move everything data-like below studenti.

c) Update signup.js according to b).
mailer.js: implement override to ease testing
backend: fix string case for name, surname
Globally switch to HTTPS

1) Make sure that we ignore the `ssl' directory in which the
private key and the certificate are saved.

2) Add the init_ssl.sh script that populates the `ssl' directory.

3) Teach server.js to use HTTPs.

4) Switch all http:// links to use https://.

5) Print the certificate fingerprint at startup.
Take periodic data snapshots using git
backend: do not exit on json parsing failure
Merge branch 'develop'