Prevent unauthorized access to views

[jdufresne]

All views in debug_toolbar are now decorated with the new
debug_toolbar.decorators.require_show_toolbar. This prevents access when
the function defined by SHOW_TOOLBAR_CALLBACK returns False. For
example, when a request originated form outside INTERNAL_IPS.

Fixes #834

[codecov]

Codecov Report

Merging #926 into master will increase coverage by 5.95%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #926      +/-   ##
==========================================
+ Coverage   77.11%   83.07%   +5.95%     
==========================================
  Files          30       31       +1     
  Lines        1643     1660      +17     
  Branches      245      246       +1     
==========================================
+ Hits         1267     1379     +112     
+ Misses        303      200     -103     
- Partials       73       81       +8

Impacted Files	Coverage Δ
debug_toolbar/panels/templates/views.py	70.21% <100%> (+46.29%)	⬆️
debug_toolbar/views.py	78.57% <100%> (+32.41%)	⬆️
debug_toolbar/panels/sql/views.py	75.8% <100%> (+62.01%)	⬆️
debug_toolbar/decorators.py	100% <100%> (ø)
debug_toolbar/middleware.py	87.5% <80%> (+10.28%)	⬆️
debug_toolbar/toolbar.py	81.42% <0%> (+1.42%)	⬆️
debug_toolbar/panels/sql/tracking.py	81.44% <0%> (+2.06%)	⬆️
debug_toolbar/panels/staticfiles.py	91.75% <0%> (+11.34%)	⬆️
... and 4 more

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