Fixed left/right arrow buttons in QC

[skymandr]

• Left/right buttons now behave as expected.
• Q-index is shown in the UI
• Added button to go to first Q-index

[codecov-io]

Codecov Report

Merging #394 into master will decrease coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #394      +/-   ##
=========================================
- Coverage   41.22%   41.2%   -0.02%     
=========================================
  Files         207     207              
  Lines       16999   16999              
  Branches     2954    2954              
=========================================
- Hits         7007    7005       -2     
+ Misses       9575    9556      -19     
- Partials      417     438      +21

Impacted Files	Coverage Δ
scanomatic/data_processing/phases/segmentation.py	73.25% <0%> (-0.56%)	:arrow_down:
scanomatic/ui_server/general.py	23.48% <0%> (ø)	:arrow_up:
scanomatic/qc/phenotype_results.py	0% <0%> (ø)	:arrow_up:
scanomatic/ui_server/data_api.py	9.65% <0%> (ø)	:arrow_up:
scanomatic/ui_server/tools_api.py	8.77% <0%> (ø)	:arrow_up:
scanomatic/data_processing/phases/features.py	17.23% <0%> (ø)	:arrow_up:
scanomatic/io/source.py	18.08% <0%> (ø)	:arrow_up:
scanomatic/data_processing/phenotyper.py	27.51% <0%> (ø)	:arrow_up:

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[skymandr]

This PR grew a little in scope. Be back later.