chore(yafti): Prepare the module for future yafti refactor

[fiftydinar]

It is not known if current yafti.yml config format will remain functional in the new refactored version of yafti.
It is not known if yafti will change it's function dramatically, instead of being the setup, being the configurator only.
It is also not known how will the new yafti look visually in terms of UI.

But it is known that there is the intention & effort in doing the refactor, which I found out in Universal Blue discord channel.

So this issue will be updated to cover further findings in this regard, to clarify if our action is needed or not.

[xynydev]

The legacy custom-flatpaks option should be removed from the module. It might be wise to move the yafti install to bling, and make the configuration the users job.

[fiftydinar]

The legacy custom-flatpaks option should be removed from the module. It might be wise to move the yafti install to bling, and make the configuration the users job.

I agree with this.

yafti is in the similar category like ublue-update, where it's the Universal Blue only component, which has potential to drastically change over time, requiring us to constantly update the template config.
So it is indeed wise to make this the user job.

[fiftydinar]

It would be good to freeze yafti version to 0.8.0 in module until we test new yafti.

Change should be simple, like this:

pip install --prefix=/usr yafti==0.8.0

Edit: here's the PR
#220

Edit2: done