Git-machete

- Improve stability of traverse and slide-out/slide-in actions.

- added: new way of acquiring the github token (from `~/.github-token`)
- fixed: `--fork-point` and `--down-fork-point` options values have to be ancestor of the current branch
- added: fish shell completions (contributed by kgadek)

- fixed: wrong logo path in Snapcraft config

- added: `github` command with `anno-prs`, `checkout-prs`, `create-pr` and `retarget-pr` subcommands
- added: documentation on readthedocs.io
- fixed: documentation displayed with `help`/`-h`/`--help`
- improved: content of git-machete project related blogs has been moved to this repo and updated
- removed: releases to Nixpkgs no longer happen directly from our CI pipeline

- Fixed a race condition in the Slide-out action.
- Fixed a `NullPointerException` in Override Fork Point action.
- Improved Override Fork Point dialog: inapplicable options are hidden, placeholders are no longer displayed.
- Fixed an error with a too long GitHub URI when reporting exceptions in the plugin.
- Fixed a too short traverse info dialog.
- Improved Slide In dialog: clarified that a branch to slide in can be created on the spot

- improved: `show` can accept a target branch other than the current branch (contributed by asford)
- added: `--no-push`, `--no-push-remote`, `--push` and `--push-untracked` flags in `traverse` to optionally skip pushing to remote (contributed by asford)