Group admin

[sohilpandya]

Added Functionality for group admin:

• Have an admin dashboard
• View all lecturers that are part of this group
• Ability to enable/disable user
• view limit of users left in this account

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• sign a user out of an admin disables their account
• custom message if a disabled user tries to log in.

Related to #623

[codecov-io]

Codecov Report

Merging #638 into staging will increase coverage by 0.21%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           staging     #638      +/-   ##
===========================================
+ Coverage    94.26%   94.47%   +0.21%     
===========================================
  Files          126      132       +6     
  Lines         2841     2951     +110     
  Branches       498      516      +18     
===========================================
+ Hits          2678     2788     +110     
  Misses         163      163

Impacted Files	Coverage Δ
src/js/reducers/user.js	100% <ø> (ø)	:arrow_up:
server/lib/group-admin/getLecturersByGroupCode.js	100% <100%> (ø)
src/js/reducers/group-admin.js	100% <100%> (ø)
server/lib/authentication/saveUser.js	100% <100%> (ø)	:arrow_up:
server/plugins/users.js	98.31% <100%> (+0.04%)	:arrow_up:
server/plugins/group-admin.js	100% <100%> (ø)
server/plugins/index.js	100% <100%> (ø)	:arrow_up:
src/js/reducers/root-reducer.js	96.77% <100%> (+0.1%)	:arrow_up:
...lib/group-admin/getGroupAccountLimitInformation.js	100% <100%> (ø)
server/plugins/authenticate-user.js	96.22% <100%> (+0.14%)	:arrow_up:
... and 9 more

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