Add scripts_end block and banner div to base layout.html

[jorisvandenbossche]

Attempt to address some customization needs for bokeh, cfr bokeh/bokeh#11130 (comment)

cc @bryevdv

[bryevdv]

@jorisvandenbossche I might suggest instead something like extra_scripts at least for the usage in Bokeh, we want to add scripts in addition to the webpack and GA parts. Unless I'm mistaken the structure above would require copy and pasting those bits in order to keep them

[jorisvandenbossche]

I think you can use super() to get the content of the parent block to be able to add something. See https://jinja.palletsprojects.com/en/2.11.x/templates/#super-blocks

[bryevdv]

Ah, in that case it seems 👍

[drammock]

This will be useful for us too, thanks!