Always use preserveComment: 'some'

While creating the `bootstrap.js` file during `grunt dist` a header gets added to that file. 

When we then create the `bootstrap.min.js` file from it, we don't specify a `preserveComments` setting in the `uglify:bootstrap` task so the default will be to strip that banner out, but because we specify an explicit `banner` setting it gets added right back in.

Therefore we could change the `banner` setting to a `preserveComments: 'some'` setting, without changing the output.

But this would result in all `uglify` tasks having a `preserveComments: 'some'` setting, so we could hoist that setting up to be a global `uglify` setting.

This PR makes those changes.

The generated `.min.js` files are identical.