Append correct extension to solutions submitted via textarea

Visual C++ refuses to compile sources with unusual extensions, like txt
Fix jury privilege check in the answer box
Extract CATS::Messages module
Move envelope_frame to CATS::Messages
Refactor envelope processing
Display envelopes only to the owner
Minor refactoring and optimization of send_message_box_frame
Use standard HTML header in answer_box
Use standard HTML header in send_message_box
Move string constants to 'lang' directory
Infrastructure for string i18n
Translate common menu items
Infrastructure for template i18n
Translate login form
Translate menu_begin and menu_end
Activate PHP syntax highlight