UI: Settings logic + functions

[royalmo]

Logic functions in settings menu:

• Take sync settings into account.
• Make it efficient (use not too much functions)
• Remove useless (for now) settings.
• Put functions in basics.js so settings can be used elsewhere.

[royalmo]

• Put also a restore default settings options.

[royalmo]

• Use 3-letter language codes instead of 2-letter in course guide, it will save time.

[royalmo]

• Think of a way to store username and passwords in a secured way (if possible), if not, in a hard-to-retreive way.