Put all config for each source into config and allow to set in config screen

[yorkshire-pudding]

At present the URL (and a few other config items) for an HTTP fetcher source is stored in feeds_source within the database in the source field and within a BLOB in the config field. There is a config file for each source but for some reason the config is split between the DB and config.
The only place to set and save the information is in the actual feeds import screen. If the source is different between environments then this can be problematic to update.

Therefore, it would be great if all config for each source could be stored in the config and be possible to set within the config.