SSC: Implement issue-template custom tag management

[rsenden]

SSC allows for manually assigning or removing custom tags to/from issue templates; we should cover this functionality in SSC either as part of the existing ssc issue-template create/update commands, or through a new command (ssc issue-template-tag?).

[MikeTheSnowman]

For mangement of custom tags in issue-templates, I think this should be managed via ssc issue-template create/update commands. The reason for this is because with the issue template manage page in SSC, you're not creating new custom tags when modifing issue templates in the web UI, you're only assigning custom tags that have already been defined.

[rsenden]

Note that command names have changed in fcli 2.x; commands are now named fcli ssc issue create-template and fcli ssc issue update-template