Rename theme, theme2, etc to actual brand names

[unekinn]

In the internal design tokens, the brand-specific tokens are in files called theme.json, theme2.json etc instead of digdir.json, altinn.json etc. Similarly, the token paths are theme.accent..., theme2.accent... instead of digdir.accent..., altinn.accent.... This is very confusing.

[mimarz]

Is this ok to rename? @Febakke

This change will affect the update:template script in the cli, but we can just make sure to pick the digdir theme for that.

[mrosvik]

@Febakke Onhold until #2674 and #2728 is done.