Fix webauthn regression and improve code (#25113)

Follow:

* #22697

There are some bugs in #22697:

* https://github.com/go-gitea/gitea/pull/22697#issuecomment-1577957966
* the webauthn failure message is never shown and causes console error
* The `document.getElementById('register-button')` and
`document.getElementById('login-button')` is wrong
    * there is no such element in code
    * it causes JS error when a browser doesn't provide webauthn
    * the end user can't see the real error message

These bugs are fixed in this PR.

Other changes:

* Use simple HTML/CSS layouts, no need to use too many `gt-` patches
* Make the webauthn page have correct "page-content" layout
* The "data-webauthn-error-msg" elements are only used to provide locale
texts, so move them into a single "gt-hidden", then no need to repeat a
lot of "gt-hidden" in code
* The `{{.CsrfTokenHtml}}`  is a no-op because there is no form
* Many `hideElem('#webauthn-error')` in code is no-op because the
`webauthn-error` already has "gt-hidden" by default
* Make the tests for "URLEncodedBase64" really test with concrete cases.


Screenshots:

* Error message when webauthn fails (before, there is no error message):

<details>


![image](https://github.com/go-gitea/gitea/assets/2114189/93cf9559-d93b-4f06-9d98-0f7032d9c65b)

</details>

* Error message when webauthn is unavailable 

<details>


![image](https://github.com/go-gitea/gitea/assets/2114189/ffc0fcd9-b93b-4418-979c-c89bb627aaf2)

</details>

templates/user/settings/security/webauthn.tmpl

@@ -1,6 +1,4 @@
-<h4 class="ui top attached header">
-{{.locale.Tr "settings.webauthn"}}
-</h4>
+<h4 class="ui top attached header">{{.locale.Tr "settings.webauthn"}}</h4>
 <div class="ui attached segment">
 	<p>{{.locale.Tr "settings.webauthn_desc" | Str2html}}</p>
 	{{template "user/auth/webauthn_error" .}}
@@ -20,7 +18,6 @@
 		{{end}}
 	</div>
 	<div class="ui form">
-		{{.CsrfTokenHtml}}
 		<div class="required field">
 			<label for="nickname">{{.locale.Tr "settings.webauthn_nickname"}}</label>
 			<input id="nickname" name="nickname" type="text" required>
@@ -29,7 +26,6 @@
 	</div>
 </div>
 
-
 <div class="ui g-modal-confirm delete modal" id="delete-registration">
 	<div class="header">
 		{{svg "octicon-trash"}}