Also add git-info and duration-info modules to the basic templates for a
better user experience.

Don't want to maintain extra code to allow zmodule to run outside zimfw.
This has no added benefit and might confuse users. If you were calling
zmodule in the shell just to get the help message, hopefully you can
look that up in the documentation instead.

Hopefully make it clearer when either the git or the degit tool is being
used.

See #535

This better allows a global instalation of Zim that can be sourced by
other users in the same machine.

See #486

Fixes #509

This reverts commit 06f8bc7.

This breaks allowing a machine to have just one init.zsh script that can
be sourced by different users.

and change initialization so ZIM_HOME is required before sourcing the
script.

This reverts commit 9a47fde.

or if it's a symlink or not a regular file. This makes more sense now
that zimfw.zsh can exist outside ZIM_HOME, that the same file can be
used my multiple users in the same machine, and that it is going to be
possible to install it with a package manager once it's stable to do so.

to zimfw.zsh or if it's a symlink or not a regular file. It does not
make sense to show a message to user saying "Run zimfw upgrade to
upgrade" if the action will then fail.

for write permission. Also, if ZIM_HOME is not defined, don't "autoload"
the zimfw function, since this is going to be a scenario where the user
only has something like `source /path/to/zim/init.zsh` in their .zshrc.

Initially the idea was to not allow this so upgrades are disabled when
the script is installed using a package manager, but this is an
unnecessary complexity. Making it less smart and more predictable.