Rename "DSV File from HTTP" adapter to "CSV File from HTTP"

## What?

Rename "DSV File from HTTP" adapter to "CSV File from HTTP".

## Why?

There is a huge overlap in functionality between our _CSV_ adapters and the _DSV File from HTTP Adapter_ (see screenshots).

The only difference (besides the HTTP fetching) seems to be the _Check Presence Only_ option.

I think CSV is the far more common term, and therefore I would suggest to rename the adapter to bring it closer to the other CSV ones.

We could also think about adding the _Check Presence Only_ field to all CSV adapters, but that could be a separate issue.

Please note that my suggestion is coming mostly from the end user perspective (regarding the naming of the adpater in the UI, and documentation). The change should not break any existing DSV adapter configurations.

## Your Environment

* Graylog Version: 5.0