Better error handling

[mattriggle05]

Add new checks to the article previewer:

A section at the top with details about image sizes.

• Uses regex to pull out images and their src
• Checks the size of the images using the headers of the returns from github

A new box in the front matter section to give errors and warnings about missing fields and incorrect types.

• Hard coded checks for every field that is used in the frontmatter
• Fields that are arrays have lists of valid values

An augmented banner section not shown when banner is not given.

• When the banner is incorrectly set up refers users back to the frontmatter errors section

I have tested everything and it all seems to be working so far.