make width of DetailsSidebar configurable
make width of DetailsSidebar configurable

Summary:
The `DetailsSidebar` has a fixed width of 300px. For some plugins it makes sense to specify a larger width for the sidebar, to show more content in it.

Usage:
```
<DetailSidebar width={500}>...</DetailSidebar>
```

Reviewed By: passy

Differential Revision: D12924525

fbshipit-source-id: 889d799017a6d3d53f6d32510863b4838a928fba

facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper