Pair programming session

We did roughly this:

• Move all @Composables out of PrivateMessageView into separate files
• Create packages contact, conversation, login and a few more to sort/group @Composables and classes
• Clean up entry point, basically got rid of UI.kt and merged it with BriarServiceImpl which is now called BriarUi
• Move application {} scope out of Main.kt into BriarUi
• Create a bunch of view models already for Login, Registration and Contacts

What's still to do:

• More view models
• Colors of introcution drawer seem broken
• Try not to use briar API calls in @Composables directly, instead move that into view models