[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/general/res/sass/user-environ/_layout.scss

@@ -100,9 +100,6 @@
             left: auto;
             width: $ueAppLogoW;
             z-index: 2;
-            &.logo-openmctweb {
-                background: url($dirImgs + 'logo-openmctweb.svg') no-repeat center center;
-            }
         }
     }
 }