Adjust Splitbox initial widths (#824)
Adjust Splitbox initial widths (#824)

* Change the initial widths of the sidebars so the editor takes up the more of the content than the sidebars.

* Change the `initialSize` props on the SplitBox to use `300px` rather than a percentage.

devtools-html/debugger.html,jbhoosreddy/debugger.html,clarkbw/debugger.html,bomsy/debugger.html,kdzwinel/debugger.html,darkwing/debugger.html,devtools-html/debugger.html,ruturajv/debugger.html,devtools-html/debugger.html,borian/debugger.html,clarkbw/debugger.html,jasonLaster/debugger.html,borian/debugger.html,bomsy/debugger.html,jbhoosreddy/debugger.html,jasonLaster/debugger.html,jbhoosreddy/debugger.html,kdzwinel/debugger.html,irfanhudda/debugger.html,tommai78101/debugger.html,jbhoosreddy/debugger.html,irfanhudda/debugger.html,jbhoosreddy/debugger.html,devtools-html/debugger.html,clarkbw/debugger.html,ruturajv/debugger.html,borian/debugger.html,darkwing/debugger.html,ruturajv/debugger.html,bomsy/debugger.html,irfanhudda/debugger.html,bomsy/debugger.html,jasonLaster/debugger.html,irfanhudda/debugger.html,jbhoosreddy/debugger.html,jasonLaster/debugger.html,wldcordeiro/debugger.html,wldcordeiro/debugger.html,tomrav/debugger.html,rrandom/debugger.html,amitzur/debugger.html,borian/debugger.html,amitzur/debugger.html,wldcordeiro/debugger.html,devtools-html/debugger.html,tommai78101/debugger.html,darkwing/debugger.html,borian/debugger.html,rrandom/debugger.html,ruturajv/debugger.html,amitzur/debugger.html,tomrav/debugger.html,tomrav/debugger.html,jbhoosreddy/debugger.html,darkwing/debugger.html,rrandom/debugger.html,amitzur/debugger.html,tomrav/debugger.html,ruturajv/debugger.html,darkwing/debugger.html,jasonLaster/debugger.html,darkwing/debugger.html,amitzur/debugger.html,clarkbw/debugger.html,wldcordeiro/debugger.html,wldcordeiro/debugger.html,jasonLaster/debugger.html,ruturajv/debugger.html,devtools-html/debugger.html,kdzwinel/debugger.html,tommai78101/debugger.html,bomsy/debugger.html,tomrav/debugger.html,jasonLaster/debugger.html,bomsy/debugger.html,ruturajv/debugger.html,clarkbw/debugger.html,kdzwinel/debugger.html,wldcordeiro/debugger.html,wldcordeiro/debugger.html,darkwing/debugger.html,rrandom/debugger.html,devtools-html/debugger.html,rrandom/debugger.html,tommai78101/debugger.html,irfanhudda/debugger.html,bomsy/debugger.html,amitzur/debugger.html,tommai78101/debugger.html,borian/debugger.html