Add hooks support for WebSub (formerly PubSubHubbub) protocol

[YuyaAbo]

resolves #1265

Note: PubSubHubbub seems to have been renamed WebSub when it was adopted by the W3C. So we use the name WebSub in our code.
ref https://github.com/pubsubhubbub/PubSubHubbub

Added support for API to subscribe and unsubscribe to repository events via WebSub protocol.

GitHub API Docs: https://docs.github.com/en/rest/webhooks#pubsubhubbub
WebSub Docs: https://www.w3.org/TR/websub/

WebSub requires Content-Type to be application/x-www-form-urlencoded, so a new factory function has been added.

This request MUST have a Content-Type header of application/x-www-form-urlencoded (described in Section 4.10.22.6 [HTML5])
https://www.w3.org/TR/websub/#subscriber-sends-subscription-request

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[codecov]

Codecov Report

Merging #2397 (bf7f3b4) into master (404bf94) will decrease coverage by 0.01%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master    #2397      +/-   ##
==========================================
- Coverage   98.06%   98.04%   -0.02%     
==========================================
  Files         119      119              
  Lines       10546    10597      +51     
==========================================
+ Hits        10342    10390      +48     
- Misses        140      142       +2     
- Partials       64       65       +1     

Impacted Files	Coverage Δ
github/github.go	97.37% <83.33%> (-0.43%)	⬇️
github/repos_hooks.go	100.00% <100.00%> (ø)

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[gmlewis]

Thank you, @YuyaAbo !
Just a couple tiny tweaks, please, then we should be ready for a second approval before merging.

[gmlewis]

Thank you, @YuyaAbo !
LGTM.

Awaiting second LGTM+Approval from any other contributor to this repo before merging.

[raynigon]

Can be merged, but it would be nice if the error case test gets added first.