[Self-Hosting] Override Fluxer branding

Problem

Currently even though we can set an instance name and various logo resources, the Fluxer branding still shows in many places, such as:

• Link embeds.
• The favicon.
• The loading screen.
• Etc.

This mainly confuses less tech literate users who already have a Fluxer account and upon seeing the name Fluxer think they need to use their Fluxer login details to get in. If the branding is consistent then this issue will be less common.

Proposal

Update the code to actually replace all instances where Fluxer name and logos are used with the options set in the instance configuration.