Consistent url for tag and keyword

I hope hackmd can provide an URL for tag and keyword. For example, I have a keyword `project1`, it would be nice to access notes of this tag with `hackmd.io?tag=project1`. There are two benefits for this. 1. The `go back` function of the browser can return to note list. 2. It is possible to create bookmarks to enable quick access.

The issue names no files or tests. Start by tracing the client-side note-list and tag/keyword navigation entry points, then determine how query parameters are handled. Done means tag and keyword views have stable URLs that support browser back navigation and bookmarks.

Scritto dal modello di indicizzazione a partire dal testo della issue.