Implemented shortcut system in the app

[DutcherNL]

Implemented new feature that allows shortcuts to be added if so desired:

e.g. app.kotkt.nl/gala

These can redirect to any url, so also external surveys and such.

[codecov[bot]]

Codecov Report

Merging #255 (3aba333) into master (b7f8bcf) will decrease coverage by 0.06%. The diff coverage is 84.00%.

@@            Coverage Diff             @@
##           master     #255      +/-   ##
==========================================
- Coverage   95.58%   95.51%   -0.07%     
==========================================
  Files         113      114       +1     
  Lines        4486     4508      +22     
  Branches      442      442              
==========================================
+ Hits         4288     4306      +18     
- Misses        128      132       +4     
  Partials       70       70              

Flag	Coverage Δ
unittests	95.51% <84.00%> (-0.07%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
squire/urls.py	100.00% <ø> (ø)
core/views.py	96.51% <76.92%> (-3.49%)	:arrow_down:
core/models.py	96.36% <85.71%> (-1.56%)	:arrow_down:
core/admin.py	92.85% <100.00%> (+0.07%)	:arrow_up:
core/shortcut_urls.py	100.00% <100.00%> (ø)

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