Manage a demo mode

[fgaudin]

Add a command which must be run every night and do :

• delete the demo account if exist
• create a demo account
• create demo data

Password change, subscription, bug report and messages must also be disallowed for this user.

[fgaudin]

Won't do that for know. Just avoid to register with "demo" account for later maybe.
It may lead to unwanted behavior to have a demo account :

• 2 users in the demo may have actions which lead to discards what the other do and one user may think there's a bug where there's not
• demo user can spam the bug tracker
• trial period is redundant with demo

[fgaudin]

Will add code to run a demo instance which do what is said before.
This instance will have only one user.

when in demo mode must disable :

• registration
• password change
• subscription
• bug report and messages
• unregister
• resend activation email

[fgaudin]

Install a demo env, fill FAQ and add a cron (each 3 hours) for :

• reset_demo_account
• cleanup