Solve the --path_prefix problem

[zhangchen-qinyinghua]

Fix a bug on implementing the --path_prefix.
The bug happens because all of the keys in the structure self.data_applications has path_prefix as their prefix.
But the function _clean_path just removes the prefix by mistake.

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[wchargin]

Hi @zhangchen-qinyinghua! Thanks for the contribution.

You’re right that the --path_prefix argument is totally broken
currently. Thanks for bringing this to our attention.

I’m not sure that it’s related to the issue that you referenced (#1176);
that issue is just a display problem that existed before the current
problems with --path_prefix were introduced (in #1512).

The code in this PR is not the right approach to solving the
--path_prefix problem. If --path_prefix is set to /test/, then
clients should be able to fetch routes like /test/data/runs. The
problem is that the frontend is currently requesting routes like
/data/runs, without the prefix. The fix is to change the frontend to
request the right routes, not to change the backend to effectively
ignore the path prefix altogether: with this change, I can fetch
/data/runs but not /test/data/runs, so --path_prefix effectively
only applies to the root route.

Let me know if this doesn’t make sense to you or you think that this
analysis is in error. Thanks again!

cc @stephanwlee

[zhangchen-qinyinghua]

Yeah, sure.
I noticed that my fixing is just kind of hacking without solving the problem essentially.
This PR seems to fix it in the right way.