[Request]: white-label support

[GeopJr]

Describe the request

I've had this idea for a while now but hadn't sat down to actually do it: white-label build support

Suppose a company or organization has their own fedi instance, for example gov.gr, and wants their employees or members to only connect to social.gov.gr, Tuba should be able to read a config on compile-time that skips the instance picking view as well as rebrand to the instance's branding

Implementation Details

• [X] This follows the GNOME HIG.

[GeopJr]

I think I'll delay this for after 0.8, literally nobody has asked for this so I'm just guessing on what they would want from it.