204 scenario description

[mschweiger]

• Added new variant of scenario description "URLDESC" to serve the scenario description in the Orbiter launchpad from a HTML or CHM file. The HELP tag in the environment block no longer overrides the description block.
• Updated most scenario descriptions to use URLDESC or HYPERDESC versions instead of plain text DESC.
• Fixed a few scenarios (dates and designations of shuttle missions for Hubble and LDEF deployment, etc.)
• Updated Html2Text and Text2Html functions in TabScenario.cpp to avoid memory leaks.
• Save page of scenario editor now provides option for separate title and text.

Closes #204