arrow-icons: Replace ⇽ , ⇾ with ← , → everywhere.

We were using these hollowed out arrows characters (⇽ , ⇾) in a few
places, these were inconsistent with the solid up and down arrow
characters (↑ , ↓) we use otherwise. This commit replaces them
everywhere in the codebase.

static/js/portico/help.js

@@ -42,8 +42,8 @@ function adjust_mac_shortcuts() {
     var keys_map = new Map([
         ['Backspace', 'Delete'],
         ['Enter', 'Return'],
-        ['Home', 'Fn + ⇽'],
-        ['End', 'Fn + ⇾'],
+        ['Home', 'Fn + ←'],
+        ['End', 'Fn + →'],
         ['PgUp', 'Fn + ↑'],
         ['PgDn', 'Fn + ↓'],
     ]);

templates/zerver/app/keyboard_shortcuts.html

@@ -194,7 +194,7 @@
                 </thead>
                 <tr>
                     <td class="definition">{% trans %}Edit your last message{% endtrans %}</td>
-                    <td><span class="hotkey"><kbd>⇽</kbd></span></td>
+                    <td><span class="hotkey"><kbd>←</kbd></span></td>
                 </tr>
                 <tr>
                     <td class="definition">{% trans %}Show message sender's profile{% endtrans %}</td>
@@ -291,7 +291,7 @@
                 </tr>
                 <tr>
                     <td class="definition">{% trans %}Switch between tabs{% endtrans %}</td>
-                    <td><span class="hotkey"><kbd>⇽</kbd> or <kbd>⇾</kbd></span></td>
+                    <td><span class="hotkey"><kbd>←</kbd> or <kbd>→</kbd></span></td>
                 </tr>
                 <tr>
                     <td class="definition">{% trans %}View stream messages{% endtrans %}</td>

templates/zerver/help/keyboard-shortcuts.md

@@ -110,7 +110,7 @@ below, and add more to your repertoire as needed.
 
 ## Message actions
 
-* **Edit last message**: `⇽` — Open the last editable message in the current
+* **Edit last message**: `←` — Open the last editable message in the current
   view (if any).
 
 ### For a selected message (outlined in blue)
@@ -165,7 +165,7 @@ Keyboard navigation (e.g. arrow keys) works as expected.
         Pressing `↑` from the first stream in the list moves
         you to the **Filter streams** input.
 
-* **Switch between tabs**: `⇽` and `⇾` — Switch between the
+* **Switch between tabs**: `←` and `→` — Switch between the
 **Subscribed** and **All streams** tabs.
 
 * **Create new stream**: `n`