Support reverse proxy providing email

[zeripath]

This PR implements #2347 by allowing the reverse proxy to set the email address on auto-register.

The current implementation does not allow for changing the email address using the reverse proxy - perhaps it should?

[techknowlogick]

Please add to example app.ini the conf value as well

[zeripath]

I knew there was somewhere else. The other values aren't in the example app.ini - I guess I should add those too

[zeripath]

I knew there was somewhere else. The other values aren't in the example app.ini - I guess I should add those too

OK that was weird - so I searched again today and the original values pop up in the app.ini.sample ... I'm not sure what happened there!

[zeripath]

@techknowlogick : I've added the appropriate lines to custom/conf/app.ini.sample

[techknowlogick]

@zeripath Look's great!

[codecov-io]

Codecov Report

Merging #5554 into master will decrease coverage by <.01%.
The diff coverage is 20%.

@@            Coverage Diff             @@
##           master    #5554      +/-   ##
==========================================
- Coverage   37.55%   37.54%   -0.01%     
==========================================
  Files         321      321              
  Lines       47172    47181       +9     
==========================================
- Hits        17714    17713       -1     
- Misses      26916    26924       +8     
- Partials     2542     2544       +2

Impacted Files	Coverage Δ
routers/admin/admin.go	0% <0%> (ø)	⬆️
modules/auth/auth.go	53.47% <0%> (-1.78%)	⬇️
modules/setting/setting.go	48.55% <100%> (+0.12%)	⬆️
modules/process/manager.go	76.81% <0%> (-4.35%)	⬇️
models/repo_list.go	63.29% <0%> (-1.27%)	⬇️
models/unit.go	14.28% <0%> (+14.28%)	⬆️

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