Added missing signals to panel by default.

[ngnpope]

I looked through the signals documentation and added some of the missing ones: m2m_changed, pre_migrate, and setting_changed.

Note that setting_changed comes under the test signals, but says that it can be imported from django.core.signals. I left out template_rendered as it is "only available during testing".

There are also django.contrib.auth.signals. I don't think adding them here would make a hard dependency on django.contrib.auth so they could be added?

[codecov]

Codecov Report

Merging #1397 into master will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1397      +/-   ##
==========================================
- Coverage   87.88%   87.81%   -0.07%     
==========================================
  Files          29       29              
  Lines        1576     1576              
  Branches      221      221              
==========================================
- Hits         1385     1384       -1     
- Misses        141      143       +2     
+ Partials       50       49       -1     

Impacted Files	Coverage Δ
debug_toolbar/panels/signals.py	86.36% <100.00%> (+4.54%)	⬆️
debug_toolbar/panels/profiling.py	86.60% <0.00%> (-2.68%)	⬇️

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[matthiask]

Thanks!