Migrate our blueprints out of tails.git and tails.boum.org

Originally created by @sajolida on #9174 (Redmine)

We want to migrate our blueprints out of our main website and in their own ikiwiki. This will allow us to:

Be able to push more documents to our blueprints (images, office documents, etc.) without polluting our main repo.
Lock down all web editing on our main website.
Allow more people to push changes through Git.
Host our website on servers that we don’t trust to push to tails.git; ideally, N>1 servers to better resist DDoS (e.g. Tor’s pool of static webservers)

Our main options are:

Migrate blueprints to a dedicated ikiwiki instance (blueprints.tails.boum.org)
Use GitLab’s wiki (#15878 (closed), https://docs.gitlab.com/ce/user/project/wiki/)

Benefits of using the GitLab wiki:

We can link to issues, MRs, commits, etc. using the GitLab link syntax
Links to issues and MRs are automatically annotated by GitLab with “(closed)”
We can manage access control with the same tools/workflow/policy as our other GitLab stuff
Arguably nicer user interface for editing pages
More powerful flavor of Markdown
Probably closer to what new contributors would expect / are used to

Benefits of using ikiwiki:

Historical contributors are used to it (whether they like it or not, is another matter, but at least they don’t have to learn anything new)
Content drafted on a blueprint can be seamlessly moved to our design doc (it does not rely on GitLab-specific syntax or features)

These subtasks assume we use a new, dedicated ikiwiki for blueprints. If we decide to instead use a GitLab wiki, they'll need to be adjusted:

Edited Oct 06, 2020 by intrigeri

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