Rationalize tags

[mhsmith]

(From #780)

mhsmith:

I've renamed the tag "first-timers-only" to "good-first-issue" to avoid potentially losing contributions from second-time contributors.

Also, what's the purpose of "up-for-grabs"? It's currently on 67 out of 80 open issues, so it isn't conveying much information. It might be better to mark those issues which are not up for grabs by assigning them to someone.

freakboy3742:

No disagreement that "first-timers-only" isn't a great name, and that our tags could definitely do with some rationalisation. However, it's not just a matter of changing the name in the Github GUI; at the very least:

• The labels definition in .github needs to be updated
• Documentation references (e.g., this one on the first time contributors's guide need to be updated
• For consistency, the same tag needs to be updated on other repos.

I've reverted this name change for now so we can have a more complete conversation about our desired end state for issue tagging across the project.