FEATURE: Adds full screen composer submit button and prompt (#17839)

Context: https://meta.discourse.org/t/still-display-the-reply-create-topic-button-when-using-full-screen-composition/123597/6?u=johani

Right now, we don't show the submit buttons when you enter the full-screen composer. The reasons for that are described in the context link above.

This PR adds the improvements highlighted here: https://meta.discourse.org/t/still-display-the-reply-create-topic-button-when-using-full-screen-composition/123597/12?u=johani

Here's a list of the changes this PR introduces:

1. When you enter full-screen mode, we will now add a prompt that matches the browser fullscreen <kbd>F11</kbd> function. It looks like so

    <img width="500" src="https://user-images.githubusercontent.com/33972521/183529813-71a20167-a661-466c-b9ef-c4d34e231000.png">
    
    The prompt fades away after a couple of seconds.
    
2. This PR adds the submit buttons to the full-screen composer mode. The submit buttons should work like normal if the post has no errors. If the post has errors (title too short, body too short, required categories/tags), then the button will make the composer exit the full-screen mode so that users will see the errors and fix them. The error logic is based on what we currently have; this PR doesn't add any new validation. Here's a video of what that looks like:

    https://meta.discourse.org/t/-/127948/14?u=johani

app/assets/javascripts/admin/addon/templates/components/composer-fullscreen-prompt.hbs

@@ -0,0 +1,3 @@
+<div class="composer-fullscreen-prompt" {{did-insert this.registerAnimationListener}}>
+  {{html-safe this.exitPrompt}}
+</div>