Bulk editing of mappings

Opening this to start more visible discussion, since it's a big one.

- select multiple mappings from a site mappings index page
    - individually without javascript
    - using header checkbox for all or shift-click for ranges with javascript
- choose to archive or redirect the selected mappings
- see a pre-save confirmation page or modal, with a new URL input if redirecting
- if all mappings are successfully updated, redirect to the last-seen site mappings index page
- if the submitted new URL is invalid, (re)display the confirmation page with all mappings and the error message
- if some mappings can't be updated for another reason (e.g. they are already invalid, probably because import bypasses validation), redisplay the confirmation page with only the ones which failed

The only acceptance criterion from the story which this doesn't address at all is client-side validation of the new URL input, but we think that is done adequately server-side for now.