Replace static PR template with bot(s)

[Jake-Shadle]

Right now our PR template includes 3 checkboxes, for doing various things, it'd be nicer to replace them with bots, such as highfivebot so that people who contribute to any of our repos only have to eg, say they've read the contribution guidelines and code of conduct once, as well as give the nice "hey, new person, thank you!" etc.

And then for more advanced stuff like, "describe your changes" could just ask them to fill out the CHANGELOG.md if they haven't, otherwise there is no need to poke them. Which would also mean that all of our repos need an actual good CHANGELOG.md in the first place which is generally a good idea for our public repos.

[arirawr]

Great idea! Lots more setup involved, but it would definitely make a better contributor experience. I can look into this in the new year.