Templating followup (for #104): Check new template name in first step and warn for invalid or potentially missing variables

[LucaBernstein]

Followup to #104

• Check for template name conflicts in first step, adapt message in second step
• Make sure that template size is not more that MAX_MSG_SIZE/2 for the start. (log as WARNING if app encounters larger one)
• Do vars check: Warn for anything potentially missing or unknown
• Automatically add space to tag field if rune before tag is not a space
• Check and possibly improve code coverage
• Think about template suggestions following /t
• Allow alias desc for description in the template vars
• Allow deletion of multiple templates at once: /t rm a b c
• Add ${tag} variable to suggestions