Allow modifying predicate schema via UI

[manishrjain]

We should allow users to set the data type, and indices on the predicates via the UI. Also, they should be able to delete the predicate and start fresh.

In addition, we can also show them some stats about the predicate on demand, i.e. number of edges, size of indices etc. We'd need to build an endpoint to get this information.

[paulftw]

Most of the features listed here have been implemented. Closing the issue, let's file separate issues for the missing features to prioritize/track independently