Add ListExternalGroupsForTeamBySlug to Teams API

[bodgit]

#2217 added support for the majority of the External Groups API but omitted listing groups for a specific Team within an Organization.

This PR adds the missing method, and corrects the comment on the original ListExternalGroups method as that actually returns all external groups within the Organization. I added the ForTeamBySlug suffix based on other methods in the source to distinguish it from the original method.

I've added tests based on the original ListExternalGroups method and everything passes.

[codecov]

Codecov Report

Merging #2674 (1f336cc) into master (4814a50) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #2674 +/- ##
=======================================
Coverage 98.05% 98.05% =======================================
Files 131 131 Lines 11442 11455 +13 =======================================
+ Hits 11219 11232 +13 
Misses 152 152 Partials 71 71 

Impacted Files	Coverage Δ
github/teams.go	98.18% <100.00%> (+0.04%)	⬆️

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[gmlewis]

Thank you, @bodgit !
LGTM.

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

[bodgit]

@gmlewis good to merge now?

[gmlewis]

Thank you, @valbeat !
Merging.