Add MembersCanForkPrivateRepositories field to Organization struct

[lmt-swallow]

This PR adds MembersCanForkPrivateRepositories field to Organization struct.

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[codecov]

Codecov Report

Merging #2389 (b510ad5) into master (697804f) will increase coverage by 0.00%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #2389 +/- ##
=======================================
Coverage 98.06% 98.06% =======================================
Files 119 119 Lines 10545 10546 +1 =======================================
+ Hits 10341 10342 +1 
Misses 140 140 Partials 64 64 

Impacted Files	Coverage Δ
github/orgs.go	96.90% <ø> (ø)
github/repos.go	98.66% <0.00%> (+<0.01%)	⬆️

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[gmlewis]

Thank you, @lmt-swallow!
Let's please shorten the field name like it's neighbors.

[gmlewis]

Thank you, @lmt-swallow !
LGTM.
Merging.