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.
Fixes: #36846
1. When there is only on OAuth2 login method, automatically direct to it
2. Fix legacy problems in code, including:
* Rename template filename and fix TODO comments
* Fix legacy variable names
* Add missing SSPI variable for template
* Fix unnecessary layout, remove garbage styles
* Only do AppUrl(ROOT_URL) check when it is needed (avoid unnecessary
warnings to end users)
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
