Welcome and news screens

To help new users onboarding, I've added a welcome screen that shows at first connection, after the Terms approval one.

Welcome screen

The welcome screen provides links to Spider documentation and news, and gives shortcuts to start with.

Once read, the screen is hidden (unless if users choses 'Remind me later').

News screens

I extended the feature to also support news.

Now, when a new user facing feature will be added, I'll create a small news screen in markdown, and it will show on next user's connection :)
Many news may be displayed in a row, with Next/Previous links.

Thus, it will be easier to keep users up-to-date!