The mechanism used for determining if the application is being served over SSL
is wrapped by the "express-session" library for "express_sid", and manual for
the "language" cookie, but it's very similar in both cases.

The "secure" flag is set if one of these is true:

1. we are directly serving Etherpad over SSL using the native nodejs
   functionality, via the "ssl" options in settings.json

2. Etherpad is being served in plaintext by nodejs, but we are using a reverse
   proxy for terminating the SSL for us;
   In this case, the user has to be instructed to properly set trustProxy: true
   in settings.json, and the information wheter the application is over SSL or
   not will be extracted from the X-Forwarded-Proto HTTP header.

Please note that this will not be compatible with applications being served over
http and https at the same time.

The change on webaccess.js amends 009b61b3, which did not work when the SSL
termination was performed by a reverse proxy.

Reference for automatic "express_sid" configuration:
https://github.com/expressjs/session/blob/v1.17.0/README.md#cookiesecure

Closes #3561.