templates: Suppress Handlebars automatic partial indentation.

Fixes #12795.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

frontend_tests/node_tests/templates.js

@@ -870,7 +870,7 @@ run_test('message_group', () => {
         {
             msg: {
                 content: 'This is message two.',
-                match_content: 'This is message <span class="highlight">two</span>.',
+                match_content: 'This is message <span class="highlight">two</span>.\n<pre>code\nblock</pre>',
                 is_stream: true,
                 unread: true,
                 id: 2,
@@ -897,7 +897,7 @@ run_test('message_group', () => {
     assert.equal(first_message_text, "This is message one.");
 
     var last_message_html = $(html).next('.recipient_row').find('div.messagebox').last().find('.message_content').html().trim();
-    assert.equal(last_message_html, 'This is message <span class="highlight">two</span>.');
+    assert.equal(last_message_html, 'This is message <span class="highlight">two</span>.\n<pre>code\nblock</pre>');
 
     var highlighted_topic_word = $(html).find('a.narrows_by_topic .highlight').text();
     assert.equal(highlighted_topic_word, 'two');

frontend_tests/zjsunit/handlebars.js

@@ -14,7 +14,10 @@ module.exports.make_handlebars = () => {
         const name = "$" + path.relative(templates_path, filename);
         if (!compiled.has(name)) {
             compiled.add(name);
-            hb.registerPartial(name, hb.compile(fs.readFileSync(filename, "utf-8"), { zjsFilename: filename }));
+            hb.registerPartial(
+                name,
+                hb.compile(fs.readFileSync(filename, "utf-8"), { preventIndent: true, zjsFilename: filename })
+            );
         }
         return name;
     };

tools/webpack.config.ts

@@ -87,6 +87,7 @@ export default (env?: string): webpack.Configuration[] => {
                             // The ones below are defined in static/js/templates.js
                             'partial', 'plural', 'eq', 'and', 'or', 'not',
                             't', 'tr'],
+                        preventIndent: true,
                     },
                 },
                 // load fonts and files