Investigate upgrading to Bootstrap 4

[Calinou]

Now that modern browsers are in widespread use, we should probably look at upgrading to Bootstrap 4. At the same time, it should modernize the theme's appearance a little.

I could look into opening a pull request to migrate to Bootstrap 4 if this is desired. This would probably have to be released in a semver-major release like 4.0.0, since some user-made customizations might break with this.

[amiyasahu]

I would love to see a pull request on this.