Remove the negative margin from `.page-content` (#29922)

The negative margin was suboptimal and presents a few unnecessary
challenges while styling the page. Remove it and add custom margin
values, which slightly changes the height a few things near the top of
the page as well:

15px less height of explore and login navbar:

<img width="899" alt="Screenshot 2024-03-20 at 00 52 34"
src="https://github.com/go-gitea/gitea/assets/115237/72a01ca4-5d17-4a0f-b915-61f95054fcb1">

15px reduced padding-top height of "user bar" and equal 4px padding
added:

<img width="484" alt="Screenshot 2024-03-20 at 00 52 50"
src="https://github.com/go-gitea/gitea/assets/115237/a8507e6d-372d-4a8b-9048-66fcf8a5facd">

3px less padding on top of repo:

<img width="552" alt="Screenshot 2024-03-20 at 00 53 49"
src="https://github.com/go-gitea/gitea/assets/115237/dede6e44-7688-440f-a1b6-13532638ae03">

templates/user/dashboard/navbar.tmpl

@@ -105,4 +105,4 @@
 	{{end}}
 	</div>
 </div>
-<div class="divider"></div>
+<div class="divider tw-mt-0"></div>

web_src/css/base.css

@@ -227,16 +227,6 @@ a.label,
   border-bottom-color: var(--color-secondary);
 }
 
-.page-content {
-  margin-top: 15px;
-}
-
-.page-content .header-wrapper,
-.page-content overflow-menu {
-  margin-top: -15px !important;
-  padding-top: 15px !important;
-}
-
 /* fix Fomantic's line-height cutting off "g" on Windows Chrome with Segoe UI */
 .ui.input > input {
   line-height: var(--line-height-default);
@@ -678,6 +668,11 @@ img.ui.avatar,
   padding-bottom: 80px;
 }
 
+.page-content.new:is(.repo,.migrate,.org),
+.page-content.profile:is(.user,.organization) {
+  padding-top: 15px;
+}
+
 /* overwrite semantic width of containers inside the main page content div (div with class "page-content") */
 .page-content .ui.ui.ui.container:not(.fluid) {
   width: 1280px;

web_src/css/dashboard.css

@@ -78,8 +78,7 @@
 }
 
 .dashboard .dashboard-navbar {
-  padding-left: 0.5rem;
-  padding-right: 0.5rem;
+  padding: 4px 12px;
 }
 
 .dashboard .dashboard-navbar .org-visibility .label {

web_src/css/repo/header.css

@@ -71,6 +71,7 @@
 }
 
 .repository .header-wrapper {
+  padding-top: 12px;
   background-color: var(--color-header-wrapper);
 }
 

web_src/css/user.css

@@ -112,6 +112,10 @@
   border: 1px solid var(--color-secondary);
 }
 
+#notification_div {
+  padding-top: 15px;
+}
+
 #notification_table {
   background: var(--color-box-body);
   border: 1px solid var(--color-secondary);