Fix image insertion behavior

[babbitt]

Various fixes related to image uploading & insertion:

Broken image uploading behavior if media is set in _config.yml

It appears that when 29b0f35 was merged, it accidentally broke image uploading if you've specified a media folder in your _config.yml.

How to reproduce

1. Edit a repo with a _config.yml that specifies a media folder as well as at least one recognized image file within config.media.
2. Attempt to upload a photo
3. src is [Object object]/{file}

The updateImageInsert function was determining path by prepending assets which is an object of all uploaded files, not the config.media path. This would result in the upload url being [Object object]/path when using the media uploader.

This fix

• Resolves this error by reinstating use of the defaultUploadPath function (reused from the default file upload code)

Un-buildable metadata as result of image form use

When an image was uploaded to/selected from the modal in the metadata section, the value was prepended with {{site.baseurl}} which is improper as jekyll will not parse liquid syntax in front matter. A side affect of fixing this behavior is that image urls hosted externally now insert properly (as requested in #842 & #1019)

How to reproduce

1. Insert an external image url eg. https://example.com/logo
2. Inserted markdown is `{{site.baseurl}}/https://example.com/logo``
   or
3. Inset an image (internal or external) via the metadata view
4. Inserted path is {{site.baseurl}}/asset

This fix

• Uses absolute paths in metadata (since Jekyll does not parse front matter)
• Checks for external urls when inserting via the toolbar (starts with http[s]:https://)

Additionally I dd the following maintenance while I was poking around

• Expanded the isImage classifier to include modern media formats based on Mozilla's Image format guide for the web
• Expanded the isMedia classifier to include formats based on Mozilla's media format guide for the web

Although I tried to spend considerable time getting testing nailed down for this change, the lack of testing infra for toolbar & assetselection made this a no go. The good news is that defaultUploadPath has tests already (which are passing)

[babbitt]

Force push to babbitt@a15b01f due to unforseen issues with es6 conflicts (use of const)