#991 ux improvements: OIDC Login Label

[DerDummePunkt]

Implements part of #991

adds env var GOTIFY_OIDC_IDP_NAME with default "OIDC"

variable is used in the UI for button labels at login and elevation forms

Few notes here:

• I decided to name the new env var GOTIFY_OIDC_IDP_NAME, since the value is used for both the login button label as well as the label of the elevation form button. Let me know if you want that variable name changed.

• I wasnt really sure what tests to add here. I could have written something like

  func TestOIDCIDPName(t *testing.T) {
   t.Setenv("GOTIFY_OIDC_IDP_NAME", "Authelia")
  
   conf, _ := Get()
  
   assert.Equal(t, "Authelia", conf.OIDC.IDPName)
   }

  in config/config_test.go, but all that would guard against is that the env variable name GOTIFY_OIDC_IDP_NAME doesnt change in the keys.go, pretty useless imho

• I havent added any length limit/check yet, I say, if some poor soul wants to put the combined works of Williams Shakespear into that button, let them. Values longer than a few chars cause a line break in the button, personally I think that still looks fine, still, if you want some arbitrary limit on that, let me know.

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.47%. Comparing base (c27a381) to head (5f23372).

Files with missing lines	Patch %	Lines
router/router.go	14.28%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1022      +/-   ##
==========================================
- Coverage   74.53%   74.47%   -0.06%     
==========================================
  Files          66       66              
  Lines        3483     3495      +12     
==========================================
+ Hits         2596     2603       +7     
- Misses        688      693       +5     
  Partials      199      199              

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