Add OAuth2 authentication (#147)

- Added Site settings > Authentication section
- Create/edit/delete your custom oauth2 configurations
- Login or signup with oauth2

app/javascript/stylesheets/components/SiteSettings/Authentication/index.scss

@@ -0,0 +1,57 @@
+.authenticationIndexPage {
+  h2 { @extend .mb-3; }
+
+  .oauthProvidersTitle {
+    @extend .d-flex;
+
+    button {
+      @extend .ml-2;
+
+      height: min-content;
+    }
+  }
+
+  .oAuthsList {
+    @extend .pl-1;
+
+    list-style: none;
+
+    .oAuthListItem {
+      @extend
+        .d-flex,
+        .justify-content-between,
+        .my-2,
+        .p-3;
+
+      .oAuthInfo {
+        @extend .d-flex;
+
+        column-gap: 32px;
+
+        .oAuthLogo { border-radius: 100%; align-self: center; }
+
+        .oAuthName { font-size: 18px; }
+      }
+      
+      .oAuthActions {
+        align-self: center;
+
+        a {
+          @extend .link;
+        }
+      }
+    }
+  }
+}
+
+.authenticationFormPage {
+  a.backButton {
+    @extend .mb-2;
+
+    font-size: 18px;
+  }
+
+  a.link {
+    @extend .link;
+  }
+}