Add support for Creating a new credential manually

[varjolintu]

Adds a new section to the extension popup that allows user to create a new credential manually. It's possible to set a specific group, or use the default when left empty. The new section is available with both unlocked/normal and login popup statuses. A separate database selection input can be added with Protocol V2.

Relevant content script code is moved to background scripts.

Fixes #1234

TODO:

• Generate a password automatically if left empty (KeePassXC side change needed)
• Hide the button with older KeePassXC versions

Screenshots or videos

Testing strategy

Manually.

Type of change

• ✅ New feature (change that adds functionality)

[varjolintu]

I'd like to merge #2347 before this. KeePassXC side related changes don't have a PR yet.