Support for full paths in remoteURL setting

__Proposal:__
remoteURL setting in the /api/v2/remotes endpoint should support full paths rather than just the host.

__Current behavior:__
I tried to deploy InfluxDB behind a proxy and forward requests for `https://api.host.com/influxdb` to `http://127.0.0.1:8086/api/v2`. I was hoping this could be achieved by setting the remoteURL parameter to `https://api.host.com/influxdb`. But what happens is that InfluxDB seems to parse this URL and only use the host part `https://api.host.com`. So to get around this I need to configure the proxy to accept `https://api.host.com/api/v2` when my desired behaviour is that I can separate influxdb calls with the prefix /influxdb.

__Desired behavior:__
The remoteURL setting should accept and call full paths when replicating.

__Alternatives considered:__
Alternative is to reserve /api/v2 entirely for InfluxDB.

__Use case:__
To be able to separate InfluxDB api calls from other api calls by using a custom prefix.

Start at the /api/v2/remotes endpoint and trace how the remoteURL setting is parsed and used during replication. Verify behavior with a remoteURL containing /influxdb, ensuring replication retains that path instead of using only the host; add or run the relevant endpoint and replication tests.

Written by the indexing model from the issue text.