Move explanation to app module

.../java/org/futo/circles/feature/circles/CirclesFragment.kt

...o/circles/feature/explanation/CirclesExplanationDialog.kt

...in/java/org/futo/circles/feature/groups/GroupsFragment.kt

app/src/main/res/drawable/explanation_circles_1.jpeg

app/src/main/res/drawable/explanation_circles_2.jpeg

app/src/main/res/drawable/explanation_circles_3.jpeg

app/src/main/res/drawable/explanation_circles_4.jpeg

app/src/main/res/drawable/explanation_groups.jpg

app/src/main/res/values/strings.xml

auth/src/main/res/values/strings.xml