Rework Oauth login buttons, swap github logo to monocolor (#24740)

Diff without whitespace:
https://github.com/go-gitea/gitea/pull/24740/files?diff=unified&w=1

- Use SVGs for GitHub and GitLab oauth providers
- Replace section wrapping with a divider
- Rework icon rendering, increase size from 32px to 40px

Before:

<img width="853" alt="Screenshot 2023-05-15 at 21 54 23"
src="6ab5cfb4-46ff-469a-bd1f-06780d4a6a0b">

After (more providers):

<img width="849" alt="Screenshot 2023-05-15 at 21 51 21"
src="fa84f92f-98e0-4aed-9357-5d62ddd98195">

<img width="856" alt="Screenshot 2023-05-15 at 21 56 45"
src="d3edd7ed-dadd-4302-aca7-08f20adc220e">


Ref: https://codeberg.org/Codeberg/Community/issues/1023

---------

Co-authored-by: Giteabot <teabot@gitea.io>

web_src/css/form.css

@@ -206,32 +206,6 @@ textarea:focus,
   }
 }
 
-.signin .oauth2 div {
-  display: inline-block;
-}
-
-.signin .oauth2 div p {
-  margin: 10px 5px 0 0;
-  float: left;
-}
-
-.signin .oauth2 a {
-  margin-right: 3px;
-}
-
-.signin .oauth2 a:last-child {
-  margin-right: 0;
-}
-
-.signin .oauth2 img {
-  width: 32px;
-  height: 32px;
-}
-
-.signin .oauth2 img.openidConnect {
-  width: auto;
-}
-
 @media (min-width: 768px) {
   .g-recaptcha-style,
   .h-captcha-style {