[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/startup-base.scss

@@ -0,0 +1,70 @@
+/*****************************************************************************
+ * Open MCT Web, Copyright (c) 2014-2015, United States Government
+ * as represented by the Administrator of the National Aeronautics and Space
+ * Administration. All rights reserved.
+ *
+ * Open MCT Web is licensed under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0.
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ * Open MCT Web includes source code licensed under additional open source
+ * licenses. See the Open Source Licenses file (LICENSES.md) included with
+ * this source code distribution or the Licensing information page available
+ * at runtime from the About dialog for additional information.
+ *****************************************************************************/
+@import "bourbon";
+@import "logo-and-bg";
+
+@mixin splashElem($m: 20%) {
+    top: $m; right: $m * 1.25; bottom: $m; left: $m * 1.25;
+}
+
+.l-splash-holder {
+    // Main outer holder.
+    @include transition-property(opacity);
+    @include transition-duration(500ms);
+    @include transition-timing-function(ease-in-out);
+    @include transition-delay(1s);
+    position: absolute;
+    top: 0;
+    right: 0;
+    bottom: 0;
+    left: 0;
+    z-index: 10000;
+    opacity: 1;
+    &.fadeout {
+        opacity: 0;
+        pointer-events: none;
+    }
+    .l-splash {
+        // The splash element.
+        @include splashElem();
+    }
+}
+
+@media only screen and (max-device-width: 767px) {
+    .l-splash-holder .l-splash {
+        @include splashElem(0);
+        border-radius: 0;
+        box-shadow: none;
+    }
+}
+
+@media only screen and (max-device-width: 767px) and (orientation: portrait) {
+    .l-splash-holder .l-splash {
+        &:before {
+            // Make the NASA logo a bit bigger when we're in portrait mode.
+            $w: 12%;
+            width: $w * 2;
+            padding-bottom: $w;
+            padding-top: $w;
+        }
+    }
+}