Allow disabling for non-Bootstrap admin theme

Problem/Motivation

External Link Pop-up module has a checkbox to allow it on admin pages (/admin/config/content/external_link_popup/settings).
If the admin theme is not Bootstrap powered while Bootstrap External Link Pop-up module is enabled, no popup is shown and a JS error is triggered in the console: "Uncaught TypeError: $(...).modal is not a function".
This error is not directly related to Bootstrap External Link Pop-up module and is expected as the admin theme is not a Boostrap one.

Steps to reproduce

1. Enable Bootstrap External Link Pop-up module
2. Check the "Show on administration pages too." option on /admin/config/content/external_link_popup/settings
3. Click on an external link of an admin page
4. nothing happens and the error is in the console

Proposed resolution

1. Add a setting page to Bootstrap External Link Pop-up module
2. Add a checkbox to disable Bootstrap External Link Pop-up module on admin page
3. Add a checkbox to load Boostrap from CDN (for ones who want to use Bootstrap modal with their non-Bootstrap admin theme)

Thanks