"Freeze" all old runs

[hroncok]

In order to make new changes safe, I porpose we freeze all runs by serving them from another branch.

My idea is to:

1. create archived2017 branch and put all runs from 2017 in there
2. create archived2018 branch and put all past runs from 2017 in there
3. make the maintainers (@encukou, @vahalova?) to use forks for the current runs (for pyladies, preferably fork to @PyLadiesCZ, I cannot do that)

Alternative would be to use separate forks for each run, but I don't think it's worth the effort.

I can proceed with 1 and 2.

[encukou]

The current run in Brno will end in a month, so we can add it to the archived2018 branch then, instead.

[hroncok]

Sure. If no changes are required.

[hroncok]

Can I proceed with 1 and 2?

[encukou]

Go ahead, please.
If the freezing API needs to change, we can touch the archived branches then.

[hroncok]

2017: #420

[hroncok]

Let's wait 10+ days and archive 2018 at once.

[encukou]

2019: #573
Issue for automation: #574