Timeline - map PlatformType to icon name

[IanMayo]

Each type of platform has a unique name. The agreed list of platform types is here: #936 (comment)

The set of icons representing these types is here: https://github.com/debrief/debrief-icons/tree/master/counters/pepys

Preview them here

We aim to show a different icon for each platform type.

But, we need to map the PlatformType to an icon filename.

To this we must:

• remove hyphens/underscores
• replace each group of spaces for a single underscore
• switch to lowercase

So, Naval - fixed wing will go to naval_fixed_wing.svg.

We should do this in the JS for timeline, IMHO.