compose: Fix bug where extra line breaks were pasted between blocks.

Due to the way turndown pads every block element with 2 new lines, and
makes `br` double space by default, we would get 3 blank lines pasted
when there's just 1 line break between 2 paragraphs.

Now we set `br` to an empty string, and since turndown collapses
sequences of multiple new lines to `\n\n` (1 blank line), so any 2 block
elements will now always have 1 blank line between them, irrespective of
if and how many line breaks there are between them in the copied HTML.

web/src/copy_and_paste.js

@@ -337,6 +337,7 @@ export function paste_handler_converter(paste_html) {
         emDelimiter: "*",
         codeBlockStyle: "fenced",
         headingStyle: "atx",
+        br: "",
     });
     turndownService.addRule("style", {
         filter: "style",

web/tests/copy_and_paste.test.js

@@ -97,6 +97,11 @@ run_test("paste_handler_converter", () => {
         "* bulleted\n* nested\n  * nested level 1\n  * nested level 1 continue\n    * nested level 2\n    * nested level 2 continue",
     );
 
+    // 2 paragraphs with line break/s in between
+    input =
+        '<meta http-equiv="content-type" content="text/html; charset=utf-8"><p>paragraph 1</p><br><p>paragraph 2</p>';
+    assert.equal(copy_and_paste.paste_handler_converter(input), "paragraph 1\n\nparagraph 2");
+
     // Pasting from external sources
     // Pasting list from GitHub
     input =