Enhancements to Mostly Static Resources (CFM-62)

[arlen]

This PR does a number of things:

• It allows mostly static resources to be cached and implements etags for them
• It provides a post_max_size check as middleware to avoid a white-screen exception if a user uploads a file that is too large for that PHP setting
• It displays image-based resources in the edit/view view
• It automatically sets an appropriate slug for the resource (with JavaScript)
• It allows relative media to be embedded while using the HtmlSanitizer library (so that resources can be used in themes)
• It allows the resource URL to be copied using our Vuejs component (this PR shows an example of embedding a Vuejs component in a fields.inc file)

Here is the edit view:

[arlen]

The force-push was to rebase against the latest develop.