Custom icons for OAuth sources

[leytilera]

This adds just a simple option, to set custom icons for the OAuth login sources. If you are using multiple OpenID Connect sources, this is very useful, because you can add icons, that actually fit to the login source and not just the default OpenID logo.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@25f8970). Click here to learn what that means.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master   #14161   +/-   ##
=========================================
  Coverage          ?   42.01%           
=========================================
  Files             ?      733           
  Lines             ?    78701           
  Branches          ?        0           
=========================================
  Hits              ?    33068           
  Misses            ?    40200           
  Partials          ?     5433           

Impacted Files	Coverage Δ
models/login_source.go	27.51% <ø> (ø)
models/oauth2.go	31.81% <0.00%> (ø)
modules/auth/auth_form.go	100.00% <ø> (ø)
routers/admin/auths.go	35.31% <0.00%> (ø)

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