Rename `templates` to `responses` in Rasa Open Source

[chkoss]

Rasa renamed the templates in the domain to responses.

In Rasa X, we've renamed all related functions, variables, API responses etc. accordingly (https://github.com/RasaHQ/rasa-x/issues/1945). We should do the same for Rasa Open Source.

[sara-tagger]

Thanks for raising this issue, @erohmensing will get back to you about it soon✨

Please also check out the docs and the forum in case your issue was raised there too 🤗

[yennycheung]

@wochinge: I took a quick look at this. It seems like we need to do a lot of changes. Could I get your help next week to walk through the code base to get some hints on which part of it needs to be renamed?

[wochinge]

@yennycheung Sure! Just send me an invite!

[yennycheung]

@wochinge, thanks, will do when I get stuck! Meanwhile, I'm trying to narrow the changes down to some flows to make the tests pass.