Add a search interface for orders on an user's admin page

[aitbw]

Description

This PR adds a search interface for orders under admin/users/:id/orders; it was a pending TODO as per this comment.

It emulates, til some point, the functionality under admin/orders. While it takes most of the fields used on that view, it does not consider others, as the modified view is limited in space and I think it's for the best to keep it as clean and concise as possible.

I also think, given the context, that admins might be not be interested in such specific details under a particular user.

Given the similarities between this view/controller action and the one for admin/orders, I think there might be room for a refactor, to DRY things up a little bit, but that'd be included on another PR if this one gets approved for Solidus.

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have updated Guides and README accordingly to this change (if needed)
• I have added tests to cover this change (if needed)

[kennyadsl]

I'd probably have removed the TODO honestly. It's a lot of duplicated code for a feature that I feel like it's not used a lot. Not sure what to do at the moment.

[aitbw]

Maybe we could apply a refactor first —extract view/action logic, make it as agnostic as possible and then add the search functionality to the related views, WDYT?