[Enhancement] Update Bootstrap to version 4.2+

[schams-net]

Problem

The application currently uses Bootstrap version 3.3.7. Further development on Bootstrap v3 has been stopped and a stable version of Bootstrap v4 was released in January 2018. The new version is a major rewrite from Bootstrapv 3 with significant changes and improvements.

Suggested Solution

Update Bootstrap to version 4.2 or newer.

Changes Required

• Remove Bootstrap v3 files from the repository.
• Include Bootstrap files from an external source.
• Rewrite HTML code to use Bootstrap v4 specific code and layout.

Risks and Dependencies

Some JavaScript/CSS library currently used possibly depend on a specific version of Bootstrap. It needs to be assessed, if they are either compatible with Bootstrap v4 or need to be updated or replaced.

Additional Notes

It is assumed, it is ok, if the Bootstrap files are included into the web interface by using their external reference (typically a CDN such as stackpath.bootstrapcdn.com and/or cloudflare.com). Subresource Integrity (SRI) should be used as a security feature in this case.

• https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity
• https://hacks.mozilla.org/2015/09/subresource-integrity-in-firefox-43/

[Pofilo]

Having external dependencies can be a real problem. For example on an instance which does not have access to the internet. My point is also valid for #233.

[BenjaminHae]

I'd be happy to accept a pr that changes the framework to bootstrap 4. But I strongly oppose loading any external resources.

[schams-net]

Understood your point re external resources. I am happy to have a look what is required to update Bootstrap to v4 and create a PR once completed and tested.

However, it makes sense to deal with that after #235 and #236 have been resolved (and are in master).

[zeruniverse]

Goto #248