Implement remove service end-point

[abn]

We need to implement the /remove// functionality in the web-app.

I propose adding a new field 'removed' in the mongo document, existence of which will indicate that the record has been removed.

{
    ...
    "removed" : DATE,
    ...
}

The other option, which will have a lesser database footprint and might be more performant, is if we create a new collection removals containing just the removal date and the respective combined sha512.

{
    "hash": "sha512_1",
    "removed": DATE
}

@ashcrow thoughts?

Since the clients work on limited subsets of data, the proposed response might be:

["sha512_1",  "sha512_2"]

This will be a sub-task of #57

[abn]

This is considered completed. Reopen if changes required.