Addons-server: Add Ko-fi.com to contributors domains

[Juraj-Masiar]

See also:
https://github.com/mozilla/addons-server/issues/13163
https://github.com/mozilla/addons-server/issues/11680

Ko-fi stands out because of it's "No platform fee". See more info about the Ko-fi here:
https://ko-fi.com/about

They have a "Gold" version that has some bonus features like monthly subscriptions.
However the free version has enough features, including:

• setting up a goal
• posting posts
• donation without registration
• donation with Credit Card (when connected through Stripe) and PayPal
• money goes directly to your account

Example page (yes, that's me!):
https://ko-fi.com/fastaddons

[eviljeff]

@jvillalobos to decide

[jvillalobos]

Looks good to me.

[jvillalobos]

Mentor: @eviljeff

[Tintinator]

Can I take this?

[eviljeff]

@Tintinator sure

[AlexandraMoga]

The new domain can be used in dev hub to set up a contribution URL for an an add-on listing:
Users will be redirected to this domain when they click on the Contribute button in the add-on detail page.

Issue verified fixed on AMO -dev.