MessageTags: Add templating & edit

[Luna-devv]

• Adds a way to put one tag into another:

# set via /tags create:
haha main

sub: [insert-tag:sub]
sub2: [insert-tag:sub2]

# when being ran:
haha main

sub: hello from sub tag
sub2: even
deeper
sub cmds

• Adds a way to edit existing tags with /tags edit
• Adds a way to view the raw tag content with /tags preview raw:true
• Improves command descriptions