Initiate design system and refactor login page buttons.

frontend/src/design_system/button.scss

@@ -0,0 +1,22 @@
+button, a.button {
+    padding: .8em 1.2em;
+    border-radius: 10em;
+    border: none;
+    background-color: #3f51b5;
+    color: white;
+    transition: background-color .2s ease-in-out;
+
+    &:hover {
+        background-color: #5b6ed8;
+        cursor: pointer;
+    }
+
+    &.secondary {
+        color: #3f51b5;
+        background-color: white;
+    
+        &:hover {
+            background-color: #f2f4ff;
+        }
+    }
+}

frontend/src/design_system/index.scss

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+@use './button.scss';