Allow feeds to accept POST requests as feed content

[engram-design]

Will also include feed mapping and all other UI settings to be able to be set via the config file, as no 'feed' exists. Requires a reasonable refactor.

[clarknelson]

+1 sounds like a big change, even though it looks simple in Guzzle

[maxstrebel]

Is this something that can be done already? I need to query a Notion Database and am a bit stuck on how to do it.