Replace package database with independently installed package repositories

- Add support for fetching and installing from package repositories containing package URLs and metadata
- Support multiple package repository sources
- Add new repo command
- Remove submit command
- Enhance describe and search commands
- Fix and improve help text for new and adjusted commands
- Fix bundle install exit status
- Refer to plugins as "plugins" instead of the general term "packages"
- Code clarity improvements
- Put plugin/theme distinction into package metadata

Docs on how to set variables in `init.fish` (#328)

Fix #328

* Ignore '.git' suffix on theme URLs.
* Regenerated the Themes doc (after a long hiatus)

* Update authors list

* Add release notes for v4

* add ukrainian translation

* fix UA translate in readme

* fix UA translation in faq

* fix UA translation in packages.md

* fix documentation link

registry: add pyenv

Fix #274 

Add dpaste

Supersedes #430