Make each mirror provide a unique virtualhost name

… in addition to dl.amnesia.b.o (that we need to ease the transition).

What we need is unique vhost names among our pool of mirrors, so our JS code can pick one for each user.

1. allocate a unique virtual hostname to each current mirror
   1. check how we allocate those names
   2. #10294 (closed)
2. update contribute/how/mirror documentation with examples for the web servers we support
3. ask all mirror operators to implement this, or whatever unique virtualhost name they prefer
4. wait for most mirrors to implement it, and at some point remove from the pool the ones who did not

Parent Task: #7161 (closed)

Subtasks

• #11054 (closed)
• #11055 (closed)
• #11056 (closed)
• #11334 (closed)
• #11335 (closed)

Related issues

• Blocked by #10294 (closed)

Original created by @intrigeri on 8635 (Redmine)