Update config to support specific browser versions

The Babel config had previously supported all browsers with greater than
0.25% global usage (according to `browserlist`). This resulted in
`babel-preset-env` including plugins sufficient to support the following
minimum browser versions:

```
{
  "chrome": "49",
  "android": "4.4",
  "edge": "16",
  "firefox": "52",
  "ios": "9.3",
  "safari": "11"
}
```

Instead, the babel config now explicitly supports chrome >= 58 and
firefox >= 53. Chrome and Firefox are the only browsers we currently
publish to, and these were the minimum versions with no additional Babel
transformations.

The minimum browser versions we support should be re-evaluated later,
after we have added tests and documentation.

The `stage-0` Babel preset has also been replaced with the specific
Babel plugins that we depend upon. We don't use most of `stage-0`, so
this allowed us to remove many unnecessary transformations. We had to
remove this preset soon anyway, because all of the stage presets are
deprecated in Babel 7.

The `stage-0` preset consisted of these plugins:
```
"transform-do-expressions"
"transform-function-bind"
"transform-class-constructor-call"
"transform-export-extensions"
"transform-class-properties"
"transform-decorators"
"syntax-dynamic-import"
"syntax-trailing-function-commas"
"transform-async-generator-functions"
"transform-async-to-generator"
"transform-exponentiation-operator"
"transform-object-rest-spread"
```

Of that list, only 'transform-class-properties' and 'transform-object-
rest-spread' were required. 'transform-async-to-generator' was being
used (as we are using async/await), but our browser targets all support
async/await, so that could be removed. The remainder of the plugins were
completely unused.

Removing some of these transformations exposed bugs in `uglify-es` that
only presented themselves in the production build. `gulp-uglify-es` has
been updated to a version that uses `terser` instead of `uglify-es`,
which has resolved these issues.

Relates to MetaMask#6805

.babelrc

@@ -5,19 +5,17 @@
       {
         "targets": {
           "browsers": [
-            ">0.25%",
-            "not ie 11",
-            "not op_mini all"
+            "chrome >= 58",
+            "firefox >= 53"
           ]
         }
       }
     ],
-    "react",
-    "stage-0"
+    "react"
   ],
   "plugins": [
     "transform-runtime",
-    "transform-async-to-generator",
-    "transform-class-properties"
+    "transform-class-properties",
+    "transform-object-rest-spread"
   ]
-}
+}