chore: Add MultiPass::{identity,profile_{picture,banner}}

[dariusc93]

What this PR does 📖

• Rename MultiPass::get_own_identity to MultiPass::identity
• Add MultiPass::profile_picture and MultiPass::profile_banner to reference to the local identity images.
• Remove Identifier::own and its variants.

Which issue(s) this PR fixes 🔨

• resolves #547

Special notes for reviewers 🗒️

Additional comments 🎤