[Frontend] Significant markup/CSS mods for splash screen

#164
https://github.jpl.nasa.gov/MissionControl/vista/issues/299
App logo and bg files renamed;
Added openmct.scss to hold openmct-specific styles;
Renamed _startup.scss to startup-base.scss
which now renders its own .css file;

Still to-do: apply to WARP including needed
mods to its About dialog screen;

platform/commonUI/about/res/templates/about-dialog.html

@@ -21,7 +21,6 @@
 -->
 <div class="abs t-about l-about t-about-openmctweb s-about" ng-controller = "AboutController as about">
     <div class="l-splash s-splash"></div>
-
     <div class="s-text l-content">
         <h1 class="l-title s-title">OpenMCT Web</h1>
         <div class="l-description s-description">