Group admin enhancement

[jessicasalmon]

• [x] added the user limit field when editing a client as per the comments in #623
• [x] changed the text in error messages as per #640

please merge after #646

[codecov-io]

Codecov Report

Merging #647 into staging will increase coverage by 0.01%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           staging     #647      +/-   ##
===========================================
+ Coverage    94.47%   94.48%   +0.01%     
===========================================
  Files          132      132              
  Lines         2951     2957       +6     
  Branches       516      519       +3     
===========================================
+ Hits          2788     2794       +6     
  Misses         163      163

Impacted Files	Coverage Δ
src/js/reducers/super-admin.js	100% <ø> (ø)	:arrow_up:
server/lib/authentication/saveUser.js	100% <100%> (ø)	:arrow_up:
server/plugins/authenticate-user.js	96.22% <100%> (ø)	:arrow_up:
server/lib/super-admin/saveClient.js	100% <100%> (ø)	:arrow_up:
server/plugins/users.js	98.31% <100%> (ø)	:arrow_up:
server/plugins/group-admin.js	100% <100%> (ø)	:arrow_up:
server/lib/validateGroupLecturerByCode.js	100% <100%> (ø)	:arrow_up:

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