Use reference tags instead of YAML anchors

This should allow us to be able to redefine `before_script` in
project-specific CI configs. An example is when templates make use of
the .thumbnail() method: the build fails unless imagemagick is
installed.

Thus we can use this in the project's CI config:

```
pages:
  before_script:
    - !reference [.apt-template, before_script]
    - apt-get install imagemagick -y
```
1 job for !9 with before-script-ref in 15 seconds (queued for 4 seconds)
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