Rename usermacro commands, add template macro commands

[pederhan]

This PR renames all *_usermacro commands to host_macro and template_macro to reflect the naming schemes used in the documentation and web interface. It also adds two new commands to define and show macros for templates.

Overview of current and new names for existing commands as well as new commands.

Current	New
define_host_usermacro	define_host_macro
show_host_usermacros	show_host_macros
show_usermacro_host_list	show_macro_hosts
show_usermacro_template_list	show_macro_templates
N/A	define_template_macro
N/A	show_template_macros

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NOTE: The old command names can still be used, but are hidden and will emit a deprecation warning when used.