Fix #25: Add date information to LM visualization

[febrezo]

Many other things have been ammended including:

• Making links larger for better readability
• Links now have a tooltip describing the action
• Colorizing links depending on the type of the event
• Selecting icons for the nodes depending on the type of system
• Some code cleaning from here and there
• Making the visualization grid reach the end of the screen
• Move to Font Awesome 4 because of problems when integrating with Visjs with Font Awesome Free 5.
• Add improved messages for TL and LM diagrams when no events have been loaded.