#641 RB - Images need an ALT field for HTML

So far I made changes to:
- Allow for an Alt Text to be entered when uploading an image
- Send that Alt Text back to the backend
- Write the Alt Text to the database as regular block content with a
token name and a value
- Handle the situation where someone just updates the alt text without
uploading an image
- Add alt text token name and default value to the html document and
block templates
- Changed the code that extracts default values to handle alt text token
names