Do not load external bootstrap resources for cdn_resources = 'in_line' | 'local'

[FieteO]

The cdn_resources option should control how external resources and stylesheets should be loaded.
Currently bootstrap is being fetched from CDN irrespective of the cdn_resources option.

Also I have observed some inconsistencies in the local option where the vis-network css and js files are not being taken from the lib folder but rather externally fetched. Please correct me here if my understanding of how these options are intended to work is wrong.

To summarise the changes:

• fetch bootstrap from CDN when cdn_resources = 'remote'
• use resources from lib/ when cdn_resources = 'local'
• remove the double heading (which is also being fixed in Removed duplicated heading in HTML output. #191)
• run example.ipynb to update the samples

[BerserkerGaruk]

Went through the commit, I agree with the changes made, but I'm going to need to verify the hash between the bootstrap and visj min.js and .min.css files on my side before merging.

[FieteO]

Do you need anything from my side for validating the checksums? @BerserkerGaruk

[FieteO]

@BerserkerGaruk @summerswallow-whi , I have rebased the branch to resolve the merge conflicts that occurred in the meantime. I would appreciate it, if you could take a look, since the MR is open for a couple of month now

[fohrloop]

Is there anything that could be done to help with this?

[wey-gu]

Thanks a lot for this fix!

[wey-gu]

I had verified e2e this change on my offline code with in_line, it worked like a charm

cc @summerswallow-whi @BerserkerGaruk

Thanks!