feat: initial datamodel + ef connection and migrations

Initial version of the data model
Includes EF initialization and migrations
EF migrations are applied on application start

TournamentOrganizer/Views/MainView.axaml

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+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             xmlns:vm="using:TournamentOrganizer.ViewModels"
+             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+             x:Class="TournamentOrganizer.Views.MainView"
+             x:DataType="vm:MainViewModel">
+  <Design.DataContext>
+    <!-- This only sets the DataContext for the previewer in an IDE,
+         to set the actual DataContext for runtime, set the DataContext property in code (look at App.axaml.cs) -->
+    <vm:MainViewModel />
+  </Design.DataContext>
+
+  <TextBlock Text="{Binding Greeting}" HorizontalAlignment="Center" VerticalAlignment="Center"/>
+</UserControl>

TournamentOrganizer/Views/MainView.axaml.cs

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+using Avalonia.Controls;
+
+namespace TournamentOrganizer.Views;
+
+public partial class MainView : UserControl
+{
+    public MainView()
+    {
+        InitializeComponent();
+    }
+}

TournamentOrganizer/Views/MainWindow.axaml

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+<Window xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:vm="using:TournamentOrganizer.ViewModels"
+        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+        xmlns:views="using:TournamentOrganizer.Views"
+        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+        x:Class="TournamentOrganizer.Views.MainWindow"
+        Icon="/Assets/avalonia-logo.ico"
+        Title="TournamentOrganizer">
+        <views:MainView />
+</Window>

TournamentOrganizer/Views/MainWindow.axaml.cs

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+using Avalonia.Controls;
+
+namespace TournamentOrganizer.Views;
+
+public partial class MainWindow : Window
+{
+    public MainWindow()
+    {
+        InitializeComponent();
+    }
+}