Introduce training introduction splash page/screen

[sudo-may]

When a user visits training for the first time in the dashboard (/progress-and-rankings/projects/<project-id>) display a splash page introducing user to that training with instructions on how to get started. Something like Welcome to <project name>, a gamified training that empowers learning by achieving fun items such as skills, levels and badges. When you ready to get started scroll down to a subject and click a "View" button to discover available skills to achieve. The message can then be followed by Take me to the training button.

Few items to note:

• Splash screen is only rendered in the SkillTree dashboard and will not be shown in skills-client displays
• Portion of the message should be customizable by a project admin
• Splash screen feature can be disabled by a project admin
• When user clicks on Take me to the training button investigate possibility of documenting this event as user's start of the training which can then be utilized in metrics
• splash page is only shown the first time user visits that training