Group data bugs

[Danwhy]

Fixes bugs reported in #644:

• Stops double count of responses
• Removes lecturer from downloaded data list
• (Hopefully) Removes scrollbar from table if not necessary

[codecov-io]

Codecov Report

Merging #660 into staging will not change coverage. The diff coverage is n/a.

@@           Coverage Diff            @@
##           staging     #660   +/-   ##
========================================
  Coverage    94.84%   94.84%           
========================================
  Files          133      133           
  Lines         3007     3007           
  Branches       524      524           
========================================
  Hits          2852     2852           
  Misses         155      155

Impacted Files	Coverage Δ
server/lib/getFullGroupData.js	100% <ø> (ø)	:arrow_up:
server/lib/group-admin/getLecturersByGroupCode.js	100% <ø> (ø)	:arrow_up:

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