[clarity/consistency] rename /explorev2/ -> /explore/ (#2802)

* rename /explorev2/ -> /explore/

* add redirect for existing explorev2 urls

* fix long line

* remove extra line

* fix missed ref in spec

superset/assets/javascripts/explore/main.css

@@ -0,0 +1,36 @@
+.scrollbar-container {
+  position: relative;
+  overflow: hidden;
+  width: 100%;
+  height: 100%;
+}
+
+.scrollbar-content {
+  position: absolute;
+  top: 0px;
+  left: 0px;
+  right: 0px;
+  bottom: 0px;
+  overflow-y: auto;
+  margin-right: 0px;
+}
+
+.fave-unfave-icon, .edit-desc-icon {
+  padding: 0 0 0 .5em;
+  font-size: 14px;
+}
+
+.checkbox {
+    float: left;
+    margin-top: 0px;
+    margin-right: 3px;
+}
+
+.control-panel-section {
+  margin-bottom: 0px;
+  box-shadow: none;
+}
+
+.background-transparent {
+  background-color: transparent !important;
+}