Ensure v5 template compatibility

[cadie]

Ensures that when the root directory templates are updated to utilize Athena, v5 templates are loaded if the current issue is set to 5. This will help with allowing us to push this version up to production before switching the current version to 6. This is the first PR in completing out #64.

Other Notes:

• Updates 404.php to use different markup based on what the current version is. Basically just removed the <big> tag and adds an Athena class for the font size. The mb-4 class was also added to the heading tag, that won't change anything on the v5 side though since no styles are attributed to it.
• Removed the get_version_front_page() function and references in the front-page.php template to it. This function was never actually getting called since we've had and will have a single page set as the homepage. get_option( 'show_on_front' ) would only not be "page" when the homepage settings are set to display posts.
• Added TODO: Athena-ize comments to ./front-page.php and ./page.php for testing purposes. These will be removed in a future PR.
• Adjusted by_version_template() so that the frontpage and page templates are looked for in the current version's directory before the root directory.
• Copied the current front-page.php template to the v5 directory. The page.php v5 template is already in the v5 directory and matches the root template.
• Removed the page.php template from the v6 directory. We will use the v6 page template from the root directory.