Escape search term before building regular expression (#5994)

When doing a conversation search, if the search term includes any regular
expression characters and the search returns results, then this function would
throw an exception.

For example, if a conversation includes the text "+15555550111" and you search
for "+15555550111" then you get an exception like:

> Invalid regular expression: /(+15555550111)/: Nothing to repeat

Because the "+" is not escaped.

app/javascript/dashboard/routes/dashboard/conversation/search/SearchMessageItem.vue

@@ -64,11 +64,16 @@ export default {
   methods: {
     prepareContent(content = '') {
       const plainTextContent = this.getPlainText(content);
+      const escapedSearchTerm = this.escapeRegExp(this.searchTerm);
       return plainTextContent.replace(
-        new RegExp(`(${this.searchTerm})`, 'ig'),
+        new RegExp(`(${escapedSearchTerm})`, 'ig'),
         '<span class="searchkey--highlight">$1</span>'
       );
     },
+    // from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions#escaping
+    escapeRegExp(string) {
+      return string.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
+    },
   },
 };
 </script>