[PLT-7342] Add function and tests to specifically determine @all & @channel (#7181)

* add function and tests to specifically determine @all & @channel

* uodate per comments - regex and tests

webapp/tests/utils/post_utils.test.jsx

@@ -0,0 +1,97 @@
+import * as PostUtils from 'utils/post_utils.jsx';
+
+describe('PostUtils.containsAtMention', function() {
+    test('should return correct @all (same for @channel)', function() {
+        for (const data of [
+            {
+                text: undefined,    //eslint-disable-line no-undefined
+                key: undefined,     //eslint-disable-line no-undefined
+                result: false
+            },
+            {
+                text: '',
+                key: '',
+                result: false
+            },
+            {
+                text: 'all',
+                key: '@all',
+                result: false
+            },
+            {
+                text: '@allison',
+                key: '@all',
+                result: false
+            },
+            {
+                text: '@ALLISON',
+                key: '@all',
+                result: false
+            },
+            {
+                text: '@all123',
+                key: '@all',
+                result: false
+            },
+            {
+                text: '123@all',
+                key: '@all',
+                result: false
+            },
+            {
+                text: 'hey@all',
+                key: '@all',
+                result: false
+            },
+            {
+                text: 'hey@all.com',
+                key: '@all',
+                result: false
+            },
+            {
+                text: '@all',
+                key: '@all',
+                result: true
+            },
+            {
+                text: '@ALL',
+                key: '@all',
+                result: true
+            },
+            {
+                text: '@all hey',
+                key: '@all',
+                result: true
+            },
+            {
+                text: 'hey @all',
+                key: '@all',
+                result: true
+            },
+            {
+                text: 'HEY @ALL',
+                key: '@all',
+                result: true
+            },
+            {
+                text: 'hey @all!',
+                key: '@all',
+                result: true
+            },
+            {
+                text: 'hey @all:+1:',
+                key: '@all',
+                result: true
+            },
+            {
+                text: 'hey @ALL:+1:',
+                key: '@all',
+                result: true
+            }
+        ]) {
+            const containsAtMention = PostUtils.containsAtMention(data.text, data.key);
+
+            expect(containsAtMention).toEqual(data.result);
+        }
+    });
+});