[feat]: Add sub-commands related to challenges

[vpchung]

Fixes (partially) #3

Changelog

Sub-commands added:

Subcommand	Description
create	Create a new challenge site

Preview

• create

  $ cnb-tools challenge create "New challenge!" -t 2
  

[pep8speaks]

Hello @vpchung! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file cnb_tools/classes/challenge.py:

Line 3:1: F401 'synapseclient.Project' imported but unused
Line 6:1: F401 'cnb_tools.classes.participant.Participant' imported but unused

• In the file cnb_tools/commands/challenge_cli.py:

Line 12:1: F401 'cnb_tools.classes.challenge.Challenge' imported but unused
Line 19:46: F722 syntax error in forward annotation 'New challenge name'
Line 23:13: F821 undefined name 'tasks_count'
Line 23:30: F821 undefined name 't'
Line 23:41: F722 syntax error in forward annotation 'Number of challenge tasks (default: 1)'
Line 29:13: F821 undefined name 'private'
Line 30:13: F821 undefined name 'p'
Line 31:18: F722 syntax error in forward annotation 'Challenge should be private, even to the Sage CNB Team (default: false)'
Line 31:89: E501 line too long (91 > 88 characters)

[sonarcloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[vpchung]

Closing this one for now, until #24 is addressed.