always show secrets tab in the main nav

[meirish]

Currently we rely on access to cubbyhole to show the secrets tab, but this isn't a very good indicator if users have access to secrets. Instead, we're just going to always show the secrets nav item and the API call will filter mounts appropriately (the API we use will only return mounts they have access to).

To test:

1. Create a policy w/o access to cubbyhole - path "secret*" { capabilities = [ "read" ] }
2. Create a token with that policy
3. Log in to the UI with that new token
4. Verify that Secrets still shows up in the main navigation

[madalynrose]

logged right into the secrets page with cubbyhole ^_^