Add default post text to onboarding flow in web UI (#24705)

app/javascript/mastodon/locales/defaultMessages.json

@@ -3211,6 +3211,10 @@
   },
   {
     "descriptors": [
+      {
+        "defaultMessage": "Hello #Mastodon!",
+        "id": "onboarding.compose.template"
+      },
       {
         "defaultMessage": "You've made it!",
         "id": "onboarding.start.title"

app/javascript/mastodon/locales/en.json

@@ -445,6 +445,7 @@
   "onboarding.actions.close": "Don't show this screen again",
   "onboarding.actions.go_to_explore": "See what's trending",
   "onboarding.actions.go_to_home": "Go to your home feed",
+  "onboarding.compose.template": "Hello #Mastodon!",
   "onboarding.follows.empty": "Unfortunately, no results can be shown right now. You can try using search or browsing the explore page to find people to follow, or try again later.",
   "onboarding.follows.lead": "You curate your own home feed. The more people you follow, the more active and interesting it will be. These profiles may be a good starting point—you can always unfollow them later!",
   "onboarding.follows.title": "Popular on Mastodon",