Ignore whitespace-only ContentWarnings.

This will break the workflow of people that use `"    "` or such as a CW
that means "beware, but I won't tell you why". :)

Author: black-puppydog

lib/depject/message/html/compose.js

@@ -278,8 +278,9 @@ exports.create = function (api) {
         })
       })
 
-      if (contentWarningInput.value) {
-        content.contentWarning = contentWarningInput.value
+      const cw = contentWarningInput.value.trim()
+      if (cw.length > 0) {
+        content.contentWarning = cw
       }
 
       try {