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