Proposal: A user interface to manage replay indexes

[machawk1]

My interpretation of what was verbally proposed by @ibnesayeed:

#213 says we will separate the admin interface from the main replay page. There were also enhancements to allow the user/admin to upload (#508) and download (#506) a CDXJ index to be interpreted by the replay system.

We should have an "index management system" to wrap these administrative functionalities in anticipation of further functionality.

In the interface, a user may do the actions in #508 and #506, potentially through drag-and-drop, as well as merge indexes for management. Eventually we might consider organizing these indexes into collections but for now, an interface to add, remove, activate, deactivate (disable) indexes for the current replay system will suffice.