Functional classroom (with stub data)

[Demigiant]

Make classroom functional with:

• Classroom selector in reserved area
• Creation of global reusable selector (that opens a popup) to choose classroom (see Global reusable popup to choose between series of string buttons #521)
• Students panel shown directly inside reserved area
• Stub of statistics

Secondary:

• Remove classroom code from editable profile (since classroom will be simply a letter)
• Add classroom code (letter) to classroom title