Fixed issue #1292; terminal scrollbar may now be scrolled

* Removed div.terminal_bg whose purpose was to hold the background SVG, as this div was occluding the scrollbar, and instead set SVG as the background-image of div.terminal_container using CSS.
* As CSS does not have a background-opacity directive, manually decoded, edited and re-encoded the SVGs (light, dark) so that all <g>s are wrapped in a top-level <g opacity="0.1">
* Improved the rules of how the background image resizes

src/app/panels/styles/terminal-styles.js

@@ -57,25 +57,10 @@ var css = csjs`
     overflow-y        : auto;
     font-family       : monospace;
     margin            : 0px;
-  }
-  .terminal_bg     {
-    display           : flex;
-    flex-direction    : column;
-    height            : 100%;
-    padding-left      : 5px;
-    padding-right     : 5px;
-    padding-bottom    : 3px;
-    overflow-y        : auto;
-    font-family       : monospace;
     background-image    : ${styles.terminal.backgroundImage_Terminal};
-    opacity           : 0.1;
-    top               : 15%;
-    left              : 33%;
-    bottom            : 0;
-    right             : 0;
-    position          : absolute;
     background-repeat   : no-repeat;
-    background-size   : 45%;
+    background-position : center 15%;
+    background-size     : auto calc(75% -  1.7em);
   }
   .terminal    {
     position: relative;

src/app/panels/terminal.js

@@ -145,8 +145,6 @@ class Terminal {
 
     self._view.term = yo`
       <div class=${css.terminal_container} onscroll=${throttle(reattach, 10)} onclick=${focusinput}>
-        <div class=${css.terminal_bg}>
-        </div>
         <div class=${css.terminal}>
             ${self._view.journal}
             ${self._view.cli}