Hint when incorrect usage is detected (refs #383)

This is one of the most common misusage of submin2-admin for new users.
The user is unaware that an environment has te be supplied (even for
help... this is very consistent, but not intuitive).

For now give a warning (and wait two seconds so the warning can be read)
and then do what the user intented.

This blocks the user from doing actions on an environment called 'help'
that is in the current directory. If you really want to do that, you can by
passing the full path (or ./help) instead of 'help'.
Improve robustness of submin2-admin (refs #383).

Check for a very common programming mistake: storage is imported but the
environment directory does not exist (ergo, the storage settings do not exist).

Show which module(s) are problematic, but do not crash. These modules should
be fixed though!
Fix broken submin2-admin modules (refs #383).

Move imports to the functions where they are used, instead of importing them
in the beginning. Importing them later (lazily?) allows us to load these
modules without a working submin environment. This is useful to show the help
(so users can figure out how everything works before creating an environment)
Disabled SSH CRUD when git is disabled (fixes #384)

Do show SSH Keys, but if git is disabled show a helpful message that this
feature is disabled because git is. If git is enabled, work as usual.
Updated licenses

Updated copyright years on submin license, found new license for git_multimail
Revert "Make sure we use the dev submin modules"

This reverts commit d9f5caf0d348e1feaa783b5a3ee60d9b519564fe.

While this might be good to prevent developer mistakes, it is bad to have
this run in production. So remove the code and we will have to find a
different solution for developers.
Updated changelog

Also changed the wording because it was ambiguous/incorrect:
('Changes since' -> 'Changes in').