Conditionally build some pages differently based on environmental variables

[harding]

This PR has three commits:

1. A new plugin which makes environmental variables (env vars) starting with BITCOINORG_ available as template variables.

2. A conditional to the robots.txt file that prevents the site from being indexed if an env var is set to "preview". This is useful for @saivann and myself who host previews of the site but don't want them to appear in search engine results. Documentation for this is also added to README.md

3. A conditional on two of the content pages that changes how they will appear on cloned versions of the site (of which there are now several besides bitcoin.com). Specifically:

   • The Bitcoin Core download page on cloned sites redirects to the language equivalent page on Bitcoin.org
   • The About Us page has all information replaced by a single sentence: "This site includes content originally published on Bitcoin.org, but it is not affiliated with Bitcoin.org."

   It is not the goal to prevent people from cloning the site but, rather, to ensure that cloned sites have accurate information about Bitcoin Core and that the contributors to Bitcoin.org don't get blamed for clone site problems.

Previews

The above commits mean the site can be compiled in three different states, so a separate preview is provided for each:

• No variables set: this is what I expect clone sites to do. Preview of the About Us page and the Download page (should automatically redirect to Bitcoin.org). The robots.txt and other site pages are unaffected.
• BITCOINORG_BUILD_TYPE=preview this is what I expect to use myself for preview builds in the future. The robots.txt denies all indexing but the About Us and Download pages are unaffected, as is the rest of the site.
• BITCOINORG_BUILD_TYPE=deployment this is what the build script will be running from now on (as will Travis). Except for inconsequential additional empty lines, all content is identical to the merge base, including the robots.txt, About Us page, and Download page

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I will update both the build server build script and the translation preview server build script at merge time.

[saivann]

@harding s/you can this/you can do this/

[saivann]

Thanks, very nice! Untested LGTM.

[saivann]

@harding Just a suggestion, maybe we could also mention "and don't forget to update instructions in the README". I don't know if that's really going to be efficient in practice but who knows.

[harding]

@saivann implemented both your comments as suggested (commits amended). Thank you for your review!

Regarding the names and emails in the README.md, I don't know how much of a problem that is. Except for Bitcoin.com, I don't think any of the other clone sites are hosting their clone source on GitHub, so nobody sees the README file.

Long-term, I think our README.md is getting awfully long and needs something like a table of contents. I had the idea of moving most of it to a toc class document similar to the dev docs with all the nice features you've added for managing long pieces of documentation. If we did that, we could mask names, emails, and links to our pull/issue trackers using the env vars.

[harding]

In the absence of critical feedback, this will be merged on Tuesday.

[harding]

Merged and build script updated. I've verified that the main site robots.txt and about us page are correct.