models, templatetags: Make tag count column in patch list optional per tag

models, templatetags: Make tag count column in patch list optional per tag

Add a field, show_column, to the Tag model to determine whether the tag
gets a tag count column in the patch list view. This allows the creation of
tags that will be collated when generating mboxes but won't take up space
in the patch list.

show_column will default to True to maintain the current behaviour by
default.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Closes: #142 ("Ability to add tags that don't also have a column in the UI")
Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Stephen Finucane <stephen@that.guru>