Improve warnings about mixed content

[isaisabel]

As a user of the ATT&CK Navigator, I want layer-links to provide better warnings/errors about mixed content requests so that it's easier to understand and debug unusable layer-links.

1. Interfaces for inputting and creating layer links should be improved with warnings when the protocol of the current page is https and the protocol of the layer link is http. Warnings should be disabled for localhost/127.0.0.1 since mixed content is allowed on localhost in chrome and firefox.
2. When loading a layer link, provide a better error message if mixed content is detected instead of the generic and misleading "no HTTP response".

See #102.

[isaisabel]

Implemented in feature/mixed-content-warning and merged into develop.

It turns out it's difficult to impossible for applications to detect when their browser is preventing requests from going through due to CORS or mixed-content issues. The improvements of this feature are therefore just documentation and error message changes which point the user to look at the console logs for more details.