feat: branding support

[GeopJr]

fix: #758

TODO:

• Implement LIMIT_TO_INSTANCE
• Modify metainfo and desktop file using the input
• Provide proper re-branding instructions (including icon)
• Test tuba with different branding
• Test how it plays alongside tuba
• Remove fields that are empty from the app
• Mention that it's rebranded in debug logs

[GeopJr]

Decided to convert LIMIT_TO_INSTANCE to DEFAULT_INSTANCE. Honestly, if someone wants to login to a different instance they can straight up edit their secret. If an org wants to 'lock' the app to a certain one they are looking for an in-house fork at that point.

edit: Though if there's a demand for this, I'd implement it. I don't have that strong of an opinion on this.