Terms of service and privacy policy feature.

If enabled, terms of service link will be shown in sign up page.

[Base for discussion on Loomio](https://www.loomio.org/d/ezlIV5rN/add-tos-and-pp-documents-for-podmins-to-use?proposal=k0LZg1M9#comment-128186).

So yeah, if enabled (default false), /terms route will work and contain either the basic template (default.haml) or custom written by podmin (terms.haml). Same templates are used for mobile too - we shouldn't duplicate these I think.

Also I'm sceptical whether there is need to allow somehow creating different language versions. Podmin can write it in whatever language they want or in different languages.

No tests yet, should probably write something for the setting <-> route functionality. Also is this as a feature something we want? Also the base template would need some discussion.

The terms are from [here](https://github.com/jaywink/terms-of-service) - though a little modified in this pull. I'm going to just delete that old repo. I've taken a print out from my local dev box for a little more convenient viewing :) [Check it out here](https://cloud.jasonrobinson.me/public.php?service=files&t=5e0e94be26f0a11b4a810d2bc72c5993).
The path should be generated by a path helper, to achieve this we usually split into two translations

```yaml
  terms: "By creating an account you accept the %{terms_link}."
  terms_link: "terms of service"
```

And call that with `t('terms', terms_link: link_to(t('terms_link'), terms_path)).html_safe`