Improve: Add functions for analysing templates

[sinclairnick]

(This is quite a broad topic, so will be ongoing.)

We should make it easier to understand various things about your templates you've written. When viewing a template, we should add a tab which explains things like:

• A list of errors that arise in the output, as per our HTML linting rules
• How many links the template has, and where they point to
• Images and their sources
• Email file size
• Suggestions for fixing problems, based on the info derived above. E.g. file size of X might affect deliverability etc.

The bulk of the work will be actually creating the email HTML analysis tools, not the creation of the UI to display it.