Closes #151.

Adds JavaScript to automatically select a file format for DocumentationItem URLs in the schema management form.

Video below shows README format automatically changing to "Markdown" when a URL ending in .md is entered, then changing to "Plaintext" when the URL changes to .txt. Next, a documentation item format similarly changing to "Plaintext" when and URL ending in .txt is entered. Finally, the user manually changes the documentation item format select, which causes further changes to the URL to be ignored.

Changes are also ignored when the URL field is initialized with a value, like when editing an existing schema.