Allow user to upload index from admin replay interface

[machawk1]

In #436 we introduced the ability to upload WARCs to the replay system from the replay interface.

In a #506 (suggestion to allow users to download the index) comment, @ibnesayeed suggested allowing a user to upload an index file using the replay admin interface, which currently resides on the main replay page (to be separated in #213).

We may also want to provide a checkbox to indicate the intent of the new index with regard to the existing index (e.g., amend or replace).

[machawk1]

Related to #505. The current implementation using the Py2-based Dockerfile relies on a sample WARC being invoked by default (#504, #310, #282).