(core) Add config to include custom CSS

Summary:
Adds a new environment variable that allows for custom
CSS to be included in all core static pages.

Test Plan: Tested manually in grist-core.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3419

app/server/lib/ICreate.ts

@@ -29,6 +29,9 @@ export interface ICreate {
   sessionSecret(): string;
   // Get configuration information to show at start-up.
   configurationOptions(): {[key: string]: any};
+  // Return a string containing 1 or more HTML tags to insert into the head element of every
+  // static page.
+  getExtraHeadHtml?(): string;
 }
 
 export interface ICreateActiveDocOptions {
@@ -90,6 +93,13 @@ export function makeSimpleCreator(opts: {
         if (config) { return config; }
       }
       return {};
-    }
+    },
+    getExtraHeadHtml() {
+      let customHeadHtmlSnippet = '';
+      if (process.env.APP_STATIC_INCLUDE_CUSTOM_CSS === 'true') {
+        customHeadHtmlSnippet += '<link rel="stylesheet" href="custom.css">';
+      }
+      return customHeadHtmlSnippet;
+    },
   };
 }

app/server/lib/sendAppPage.ts

@@ -90,10 +90,12 @@ export function makeSendAppPage(opts: {
     const tagManagerSnippet = needTagManager ? getTagManagerSnippet(process.env.GOOGLE_TAG_MANAGER_ID) : '';
     const staticOrigin = process.env.APP_STATIC_URL || "";
     const staticBaseUrl = `${staticOrigin}/v/${options.tag || tag}/`;
+    const customHeadHtmlSnippet = server?.create.getExtraHeadHtml?.() ?? "";
     const warning = testLogin ? "<div class=\"dev_warning\">Authentication is not enforced</div>" : "";
     const content = fileContent
       .replace("<!-- INSERT WARNING -->", warning)
       .replace("<!-- INSERT BASE -->", `<base href="${staticBaseUrl}">` + tagManagerSnippet)
+      .replace("<!-- INSERT CUSTOM -->", customHeadHtmlSnippet)
       .replace("<!-- INSERT CONFIG -->", `<script>window.gristConfig = ${JSON.stringify(config)};</script>`);
     resp.status(options.status).type('html').send(content);
   };