Feat/refactor branding

this is done on top of the sip-login - review will be easier when rebased on top of that.

for the record (as will be obvious when reading), i'm doing the absolutely minimal changes to switch the provider seeding from the go backend to frontend-injection. this implies that the whole pkg.config.Flags approach can be re-thinked - and probably should, into something more nice like a struct that holds a map with the info for several providers etc - that is the only thing missing to open the possibility of a multi-provider build for libraries (or riseup/calyx bitmask etc).