Avoid using `StateEnum` index for Firestore

[nilsreichardt]

It's easy to change the order of the StateEnum items and this will break a few things, e.g. user will have a different state selected. Instead we should use:

int toFirestoreIndex() {
    return switch (this) {
      StateEnum.badenWuerttemberg => 0,
      StateEnum.bayern => 1,
      StateEnum.berlin => 2,
      StateEnum.brandenburg => 3,
      StateEnum.bremen => 4,
      StateEnum.hamburg => 5,
      StateEnum.hessen => 6,
      StateEnum.mecklenburgVorpommern => 7,
      StateEnum.niedersachsen => 8,
      StateEnum.nordrheinWestfalen => 9,
      StateEnum.rheinlandPfalz => 10,
      StateEnum.saarland => 11,
      StateEnum.sachsen => 12,
      StateEnum.sachsenAnhalt => 13,
      StateEnum.schleswigHolstein => 14,
      StateEnum.thueringen => 15,
      StateEnum.notFromGermany => 16,
      StateEnum.anonymous => 17,
      StateEnum.notSelected => 18,
      StateEnum.burgenland => 19,
      StateEnum.kaernten => 20,
      StateEnum.niederoesterreich => 21,
      StateEnum.oberoesterreich => 22,
      StateEnum.salzburg => 23,
      StateEnum.steiermark => 24,
      StateEnum.tirol => 25,
      StateEnum.vorarlberg => 26,
      StateEnum.wien => 27,
      StateEnum.aargau => 28,
      StateEnum.appenzellAusserrhoden => 29,
      StateEnum.appenzellInnerrhoden => 30,
      StateEnum.baselLandschaft => 31,
      StateEnum.baselStadt => 32,
      StateEnum.bern => 33,
      StateEnum.fribourg => 34,
      StateEnum.geneva => 35,
      StateEnum.glarus => 36,
      StateEnum.graubuenden => 37,
      StateEnum.jura => 38,
      StateEnum.luzern => 39,
      StateEnum.neuchatel => 40,
      StateEnum.nidwalden => 41,
      StateEnum.obwalden => 42,
      StateEnum.schaffhausen => 43,
      StateEnum.schwyz => 44,
      StateEnum.solothurn => 45,
      StateEnum.stGallen => 46,
      StateEnum.thurgau => 47,
      StateEnum.ticino => 48,
      StateEnum.uri => 49,
      StateEnum.valais => 50,
      StateEnum.vaud => 51,
      StateEnum.zug => 52,
      StateEnum.zurich => 53,
    };
  }

  static StateEnum fromFirestoreIndex(int index) {
    return switch (index) {
      0 => StateEnum.badenWuerttemberg,
      1 => StateEnum.bayern,
      2 => StateEnum.berlin,
      3 => StateEnum.brandenburg,
      4 => StateEnum.bremen,
      5 => StateEnum.hamburg,
      6 => StateEnum.hessen,
      7 => StateEnum.mecklenburgVorpommern,
      8 => StateEnum.niedersachsen,
      9 => StateEnum.nordrheinWestfalen,
      10 => StateEnum.rheinlandPfalz,
      11 => StateEnum.saarland,
      12 => StateEnum.sachsen,
      13 => StateEnum.sachsenAnhalt,
      14 => StateEnum.schleswigHolstein,
      15 => StateEnum.thueringen,
      16 => StateEnum.notFromGermany,
      17 => StateEnum.anonymous,
      18 => StateEnum.notSelected,
      19 => StateEnum.burgenland,
      20 => StateEnum.kaernten,
      21 => StateEnum.niederoesterreich,
      22 => StateEnum.oberoesterreich,
      23 => StateEnum.salzburg,
      24 => StateEnum.steiermark,
      25 => StateEnum.tirol,
      26 => StateEnum.vorarlberg,
      27 => StateEnum.wien,
      28 => StateEnum.aargau,
      29 => StateEnum.appenzellAusserrhoden,
      30 => StateEnum.appenzellInnerrhoden,
      31 => StateEnum.baselLandschaft,
      32 => StateEnum.baselStadt,
      33 => StateEnum.bern,
      34 => StateEnum.fribourg,
      35 => StateEnum.geneva,
      36 => StateEnum.glarus,
      37 => StateEnum.graubuenden,
      38 => StateEnum.jura,
      39 => StateEnum.luzern,
      40 => StateEnum.neuchatel,
      41 => StateEnum.nidwalden,
      42 => StateEnum.obwalden,
      43 => StateEnum.schaffhausen,
      44 => StateEnum.schwyz,
      45 => StateEnum.solothurn,
      46 => StateEnum.stGallen,
      47 => StateEnum.thurgau,
      48 => StateEnum.ticino,
      49 => StateEnum.uri,
      50 => StateEnum.valais,
      51 => StateEnum.vaud,
      52 => StateEnum.zug,
      53 => StateEnum.zurich,
      _ => StateEnum.notSelected,
    };
  }