Properly handle static assets for announcements

[ghost]

Description

This PR allows dispatcher to handle static assets (such as images) for announcements. To use dispatcher for this, you can use {{DISPATCH_PUBLIC}} as a domain name in GameAnnouncement.json and GameAnnouncementList.json.

In addition, this PR:

• Properly determine MIME of served file
• Properly handle 404 when file does not exist or is a directory
• Ensure bytes are converted to String as UTF-8, otherwise Java fallback is system locale
• Removes routes handlers for /sw.js and /dora/lib/vue/2.6.11/vue.min.js. The first is not necessary nor ever called with cleared UnityWebView cache and second is something specific that we do not need (in case you wish to use certain files instead of writing HTML yourself, just replace that URL with /hk4e/announcement/vue.min.js).

Both tag_icon.png and banner images not provided for obvious reasons.

Issues fixed by this PR

N/A

Type of changes

• Bug fix
• New feature
• Enhancement
• Documentation

Checklist:

• My code follows the style guidelines of this project
• My pull request is unique and no other pull requests have been opened for these changes
• I have read the Contributing note and Code of conduct
• I am responsible for any copyright issues with my code if it occurs in the future.