Add copyright notice config and apply to files

.idea/copyright/agpl.xml

.idea/copyright/profiles_settings.xml

.idea/scopes/briar_desktop.xml

HACKING.md

build.gradle.kts

buildSrc/build.gradle.kts

...rproject/briar/desktop/builddata/AbstractBuildDataTask.kt

...g/briarproject/briar/desktop/builddata/BuildDataPlugin.kt

...oject/briar/desktop/builddata/BuildDataPluginExtension.kt

...n/org/briarproject/briar/desktop/builddata/FileBuilder.kt

...ct/briar/desktop/builddata/GenerateBuildDataSourceTask.kt

...n/kotlin/org/briarproject/briar/desktop/builddata/Util.kt

src/main/kotlin/androidx/compose/material/TextFieldExt.kt

.../kotlin/org/briarproject/briar/desktop/BriarDesktopApp.kt

...n/org/briarproject/briar/desktop/DesktopDatabaseConfig.kt

...lin/org/briarproject/briar/desktop/DesktopFeatureFlags.kt

...in/kotlin/org/briarproject/briar/desktop/DesktopModule.kt

src/main/kotlin/org/briarproject/briar/desktop/Main.kt

...project/briar/desktop/attachment/media/ImageCompressor.kt

...ect/briar/desktop/attachment/media/ImageCompressorImpl.kt