Added blocks for more flexibility with JS

[ghost]

Added blocks for more flexibility. Specially the ability to create scripts in templates after loading jQuery.
This will allow to include extra js libraries that are large and not needed in every single page. Like openlayers for example.
And also include extra js for single templates.

[jtauber]

Welcome @jucajuca !

These seem reasonable (although we've oscillated about extra_scripts versus overriding scripts and using super).

One thing that would be really handy (in general, doesn't have to be done in this PR) is documentation on the blocks in PTB. I frequently find myself having to go to base.html to see what they are and this just added to that challenge.

[ghost]

I see % scripts % more for site wide scripts.
% extra_scripts % would for template specific scripts.
Block.super seems to be also a good solution, but this adds more flexibility. I don't know visiting base.html is almost a must for the development. I ended up overriding it to include these new blocks.

[ghost]

actually I think that base.html should be copied to the templates of the project. There is no point in hiding it under project/venv/lib/pythonx.x/site-packages/.../.../......
would make things much easier and deliver the same result