Add a new property of Crowd class

[ChowRex]

• Fix lxml CVE, using bs4 to replace.
• Add a property for Crowd class to retrieves full details of all group memberships.

[ChowRex]

Hi @gonchik !
As your said early (#1400 (comment)), I fixed this CVE by using bs4.

[codecov]

Codecov Report

Attention: Patch coverage is 23.07692% with 10 lines in your changes missing coverage. Please review.

Project coverage is 34.37%. Comparing base (b81fff5) to head (65f655c).
Report is 14 commits behind head on master.

Files	Patch %	Lines
atlassian/crowd.py	23.07%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1402      +/-   ##
==========================================
- Coverage   34.39%   34.37%   -0.02%     
==========================================
  Files          46       46              
  Lines        8569     8582      +13     
  Branches     1598     1601       +3     
==========================================
+ Hits         2947     2950       +3     
- Misses       5499     5509      +10     
  Partials      123      123              

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[gonchik]

@ChowRex Thank you!