Fix#433
I added the repo link to Cargo.toml so that `software.repository` could be configurable like @rhaamo suggested. I don't know if it's ok to include `software.repository` without bumping the schema version, but I didn't know if that would break any clients that parse nodeinfo with a hardcoded schema version.
This pull request add a `categories` key to the web manifest. The categories key was [recently added](https://www.aaron-gustafson.com/notebook/categories-land-in-the-web-app-manifest/ "Blog post detailing the addition of categories to the spec") to the Web Manifest spec and is a list of categorizations you want to apply to your site that serves as a hint to search engines and webapp catalogs.
The categories can be anything and there is no standardized list of vaues. The W3C is maintaining a list of commonly used values [here](https://github.com/w3c/manifest/wiki/Categories). I added _social_ to Plume's manifest based on that list, but I think eventually allowing a user defined list would be better. I don't know if there's anything in Plume currently that would enable that.