Fix URL related escaping for oauth2 (#37334)

Follow up #37327. See the comments.

* Root problem: the design of OAuth2 providers is a mess, the display
name is used as provider's name and used in the URL directly
* The regressions:
* When trying to fix https://github.com/go-gitea/gitea/issues/36409 , it
introduced inconsistent URL escaping for the "path" part.
* This fix: always use "path escaping" for the path part, add more tests
to cover all escaping cases.

Now, frontend "pathEscape" and "pathEscapeSegments" generate exactly the
same result as backend.

templates/user/auth/external_auth_methods.tmpl

@@ -1,7 +1,6 @@
 <div id="external-login-navigator" class="tw-py-1 tw-flex tw-flex-col tw-gap-3">
 	{{range $provider := .OAuth2Providers}}
-		{{/* use QueryEscape for consistent with frontend urlQueryEscape, it is right for a path component */}}
-		<a class="ui button external-login-link tw-gap-3" data-require-appurl-check="true" rel="nofollow" href="{{AppSubUrl}}/user/oauth2/{{$provider.DisplayName | QueryEscape}}">
+		<a class="ui button external-login-link tw-gap-3" data-require-appurl-check="true" rel="nofollow" href="{{AppSubUrl}}/user/oauth2/{{$provider.DisplayName | PathEscape}}">
 			{{$provider.IconHTML 24}} {{ctx.Locale.Tr "sign_in_with_provider" $provider.DisplayName}}
 		</a>
 	{{end}}

templates/user/settings/security/accountlinks.tmpl

@@ -9,7 +9,7 @@
 				<div class="menu">
 					{{range $key := .OrderedOAuth2Names}}
 						{{$provider := index $.OAuth2Providers $key}}
-						<a class="item" href="{{AppSubUrl}}/user/oauth2/{{$key}}">
+						<a class="item" href="{{AppSubUrl}}/user/oauth2/{{$key | PathEscape}}">
 							{{$provider.IconHTML 20}}
 							{{$provider.DisplayName}}
 						</a>