Use improved enterprise "get users" endpoint, and simplify collaborations model

[mesozoic]

Two changes here:

1. Calling enterprise.user() or enterprise.users() now uses the get users by id or email endpoint, which used to only accept emails (at least, according to its documentation). This removes the need to iterate over IDs and call get user by id one by one.
2. When retrieving a user or group, we will now request collaborations by default. This can be disabled by passing collaborations=False as a keyword argument to user(), users(), or group(). This removes the need for type-checked code to do if user.collaborations: ... to check against None.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (256812c) 99.61% compared to head (7159c67) 99.61%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #332   +/-   ##
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  Coverage   99.61%   99.61%           
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  Files          24       24           
  Lines        1801     1803    +2     
=======================================
+ Hits         1794     1796    +2     
  Misses          7        7           

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