Remove fomantic site module (#29980)

Had to fiddle a bit with the css ordering, but seems to work well now
and should render exactly like before. Some of the CSS may be
unnecessary, but I kept it for now.

web_src/css/base.css

@@ -37,6 +37,23 @@
   border-color: currentcolor;
 }
 
+html, body {
+  height: 100%;
+  font-size: 14px;
+}
+
+body {
+  line-height: 1.4285rem;
+  font-family: var(--fonts-regular);
+  color: var(--color-text);
+  background-color: var(--color-body);
+  tab-size: var(--tab-size);
+  display: flex;
+  flex-direction: column;
+  overflow-x: visible;
+  overflow-wrap: break-word;
+}
+
 textarea {
   font-family: var(--fonts-regular);
 }
@@ -60,13 +77,65 @@ h6 {
   font-weight: var(--font-weight-semibold);
 }
 
-body {
-  color: var(--color-text);
-  background-color: var(--color-body);
-  tab-size: var(--tab-size);
-  display: flex;
-  flex-direction: column;
-  overflow-wrap: break-word;
+h1,
+h2,
+h3,
+h4,
+h5 {
+  line-height: 1.28571429;
+  margin: calc(2rem - 0.1428571428571429em) 0 1rem;
+  font-weight: var(--font-weight-medium);
+  padding: 0;
+}
+
+h1 {
+  min-height: 1rem;
+  font-size: 2rem;
+}
+
+h2 {
+  font-size: 1.71428571rem;
+}
+
+h3 {
+  font-size: 1.28571429rem;
+}
+
+h4 {
+  font-size: 1.07142857rem;
+}
+
+h5 {
+  font-size: 1rem;
+}
+
+h1:first-child,
+h2:first-child,
+h3:first-child,
+h4:first-child,
+h5:first-child {
+  margin-top: 0;
+}
+
+h1:last-child,
+h2:last-child,
+h3:last-child,
+h4:last-child,
+h5:last-child {
+  margin-bottom: 0;
+}
+
+p {
+  margin: 0 0 1em;
+  line-height: 1.4285;
+}
+
+p:first-child {
+  margin-top: 0;
+}
+
+p:last-child {
+  margin-bottom: 0;
 }
 
 table {
@@ -121,8 +190,8 @@ progress::-moz-progress-bar {
 }
 
 ::selection {
-  background: var(--color-primary-light-1) !important;
-  color: var(--color-white) !important;
+  background: var(--color-primary-light-1);
+  color: var(--color-white);
 }
 
 ::placeholder,
@@ -146,6 +215,7 @@ progress::-moz-progress-bar {
 a {
   color: var(--color-primary);
   cursor: pointer;
+  text-decoration: none;
   text-decoration-skip-ink: all;
 }