Updated and standardized code and file structure to match current SN standards.

[xthursdayx]

I updated and standardized the directory structure and code and created a tagged release to allow for the inclusion of this theme in the Standard Notes Extensions Self-Hosted Repository. I'll be happy to correct thee repo links in the ext.json file (and elsewhere) if this PR is merged (replacing xthursdayx, which is there temporarily to get this repo to work with SN, with ilindaniel.