Refactor category admin commands. Closes #39, #40, #41.

[cowboy]

I'm re-implementing PR #56 because a lot has changed since that was started.

This PR:

• Adds a skill_category-specific is_active migration (skill will be done in a future PR)
• Renames the existing in-progress category admin commands to the following:
  • admin category add
  • admin category update
  • admin category activate
  • admin category deactivate
• Adds the following commands, because they seemed helpful:
  • admin category list - List all skill categories.
  • admin category info - Get information about a category.
• Modifies the list command to:
  • show still-active skills in deactivated categories under a Uncategorized header

The following commands have been implemented:

• [x] admin category list
• [x] admin category info
• [x] admin category add (closes #39)
• [ ] admin category update (not done yet)
• [x] admin category activate (closes #41)
• [x] admin category deactivate (closes #40)

Question:

• Can I change the terminology from activate/activated and deactivate/deactivated to enable/enabled and disable/disabled?

That would change the following:

• admin category activate would become admin category enable
• admin category deactivate would become admin category disable
• (plus a handful of newly-added strings would change)

[cowboy]

Feel free to test by messaging /dm skillsbot_cowboy help admin, etc

[cowboy]

@tkellen any last comments before i merge this?

[tkellen]

I think we covered everything in our conversation yesterday. I captured an approach for some of the concerns in https://github.com/bocoup/skillsbot/issues/83

[tkellen]

:+1: