Improvements to publishing with automatic PR creation (#1398)

Improvements to publishing with automatic PR creation (#1398)

* Added --submit switch to `alr publish`

* Forking of community index

* Changes up to pushing the manifest

* PR creation

* Reshaped as steps of the assistant

* Use release-specific branch

* Report PR webpage after creation

* Detect existing conflicting PR

* Minor tweaks (draft mode, texts, busy spinners)

* Documentation

* Allow overriding of community index info

This will allow our testing and help maintainers of other indexes

* Fix temp dir being not absolute

* Fixes for self-build with `alr build`

* Fixes for Windows temp file specifics

* Document all steps more thoroughly

Also use a better name for the new child package.

* Make submission the default after user confirmation

Document this change in behavior and the new `--skip-submit` switch that
restores the former behavior.

* Default to submit, and add switch not to submit