swap order of completion instruction

[davidism]

The order was previously source_{shell} and complete_{shell} because support for other shells was added later. In the new system the shell is always required, there is no longer the assumption of Bash if none is given. Since we've rewritten the system and all existing completion instructions and scripts are invalid anyway, take the opportunity to move the shell first. The order is now {shell}_source and {shell}_complete.

One potential argument for the old order is that it could be used for autodetection. If the shell was left off the end, detect the shell. However, autodetection isn't particularly useful since the scripts and activation instruction differ between shells anyway. If we do want that later, being explicit with auto_source is fine too.

related to #1484

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.