Following channel with fediverse address not working

/label Type: Bug Status: Need triage

Steps to reproduce

• Go to Channels > Add new
• Give a fediverse adress, for instance: @gutsud@open.audio

What happens?

Channel is not followed, following error message pops up: Error while fetching object. Enter a valid URL.

What is expected?

Channel is followed

Context

Funkwhale version(s) affected: v1.0.1

• Firefox Web Console:

XHR POSThttps://funkwhale.example.com/api/v1/channels/rss-subscribe/
[HTTP/1.1 400 Bad Request 94ms]

	
POST
	https://funkwhale.example.com/api/v1/channels/rss-subscribe/
Status400
Bad Request
VersionHTTP/1.1
Transferred633 B (30 B size)

    	
    access-control-allow-credentials
    	true
    access-control-allow-origin
    	https://funkwhale.example.com
    allow
    	POST, OPTIONS
    Connection
    	keep-alive
    Content-Length
    	30
    Content-Type
    	application/json
    Date
    	Sat, 21 Nov 2020 22:39:25 GMT
    retry-after
    	3093
    Server
    	nginx/1.14.2
    vary
    	Cookie, Origin
    x-content-type-options
    	nosniff
    x-frame-options
    	DENY
    x-ratelimit-duration
    	3600
    x-ratelimit-limit
    	2000
    x-ratelimit-remaining
    	1991
    x-ratelimit-reset
    	1606001965
    x-ratelimit-resetseconds
    	3600
    x-ratelimit-scope
    	authenticated-wildcard
    x-xss-protection
    	1; mode=block
    	
    Accept
    	application/json, text/plain, */*
    Accept-Encoding
    	gzip, deflate, br
    Accept-Language
    	en-US,en;q=0.5
    Connection
    	keep-alive
    Content-Length
    	28
    Content-Type
    	application/json;charset=utf-8
    Cookie
    	csrftoken=xxxxxx; sessionid=xxxxx
    DNT
    	1
    Host
    	funkwhale.jovuit.site
    Origin
    	https://funkwhale.example.com
    Referer
    	https://funkwhale.example.com/subscriptions
    User-Agent
    	Mozilla/5.0 (X11; Linux x86_64; rv:83.0) Gecko/20100101 Firefox/83.0
    X-CSRFToken
    	xxxxxxxxxxxxx

• Funkwhale Server logs: Nov 21 23:41:53 raspberrypi gunicorn[478]: 2020-11-21 22:41:53,846 django.request WARNING Bad Request: /api/v1/channels/rss-subscribe/