Refactor login page (#31530)

As requested in
https://github.com/go-gitea/gitea/pull/31504#issuecomment-2196196646.
This PR refactor the login page:

![Screenshot from 2024-07-04
19-23-10](c45700f4-6747-473c-bdee-2156718a7953)

![Screenshot from 2024-07-04
19-23-21](b1bf71cf-85f9-4517-a409-cc6d72e6af8f)

![Screenshot from 2024-06-30
09-35-20](728cc37e-0cca-4883-afec-a43663d2c666)
- [x] use separate box for passkey login and go to registration
- [x] move forgot passoword next to password label
- [x] fix password required label `*` and padding
- [x] remove tabs from login page

---------

Co-authored-by: silverwind <me@silverwind.io>
(cherry picked from commit 9c00dda33ad7bd594dd8f331fa1bf0acefc2ca54)

Conflict resolution:
* signin_navbar.tmpl has been deleted
* additions in form.css
* major code changes in signin_openid
* ...

web_src/css/form.css

@@ -448,6 +448,24 @@ textarea:focus,
   }
 }
 
+/* form fields with additional content besides their label, used on login form
+ * use like <div class="field"><label/><a/><input/></div> */
+.form-field-content-aside-label {
+  display: grid;
+  grid-template-columns: 1fr 1fr;
+}
+.form-field-content-aside-label > *:nth-child(2) {
+  text-align: right;
+  margin-bottom: 4px;
+}
+.form-field-content-aside-label input {
+  grid-column: span 2;
+}
+
+.ui.form .field > .selection.dropdown {
+  min-width: 14em; /* matches the default min width */
+}
+
 .new.webhook form .help {
   margin-left: 25px;
 }