[INFRA] Add watermark to drafts

[effigies]

Addresses #373.

Created a mkdocs plugin that enables overriding config options on a per-branch basis: https://github.com/effigies/mkdocs-branchcustomization-plugin

Set the plugin to add a stylesheet to every branch but master. I pushed to the main repository and enabled RTD for this branch, so we can see it: https://bids-specification.readthedocs.io/en/enh-watermark/

Things that need doing:

• ReadTheDocs adds CSS for their version panel that this obliterates. Fix that.
• Test with tags to make sure that we aren't going to mark all releases as draft.
• Think about styling.
• Consider: Do we want two or three cases? We currently have master/other. We could have release/latest/other.

[sappelhoff]

This looks like a very well implemented and easy to maintain solution. Thanks a lot @effigies.

re: style --> I like how it is: It's an obvious marker with the combination of watermark AND colorchange

re: more styles --> I think that one style is enough and that (at least currently) we don't need more styles. In the future, this would boil down to simply adding a CSS file and adjusting a line for your mkdocs plugin, right?

[effigies]

In the future, this would boil down to simply adding a CSS file and adjusting a line for your mkdocs plugin, right?

Yes. Something like:

- branch: /^master$/
  extra_css:
    - style/unreleased.css
- branch: /(?!^master$)/
  extra_css:
    - style/draft.css

[KirstieJane]

I think this looks awesome!

[effigies]

Cool. Thanks for the reviews. I'll go ahead and merge so I can include it in common-derivatives and derivatives,

[effigies]

In the future, this would boil down to simply adding a CSS file and adjusting a line for your mkdocs plugin, right?

Actually, I think that what I described above would work on any tag except ones that are on the latest master commit. We'd need to update the plugin to handle tags.