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).
Given that a lot of pods are called 'diaspora*' in `AppConfig.settings.pod_name`, should we use the pod domain rather than pod name in these terms, to avoid confusion?

A second question is: should we add 'the diaspora* network' to the pod name here? That is more of a legal question though, so something to consider as and when we have access to some legal expertise.