Implement a front end using Elm and markdown-it

[rriegs]

The simplest front end need only enable the user to perform three things:

• To load and view existing topics
• To navigate between topics
• To create and edit topics

This front end will load multiple topics for simultaneous viewing via Ajax GET requests triggered by clicking links to URL fragments (i.e. paths starting with #). Loaded topics will be inserted into a vertical list of divs with content rendered client-side via markdown-it. Each div will feature buttons to Edit and Close their respective topics. Upon editing, changes will be rendered live, and buttons will be replaced with options to Delete, Save, or Cancel changes. Intuitively, the Delete button will issue DELETE requests while Save will issue PUT. Attempting to load a non-existent topic will instead open the topic for editing.

[rriegs]

The above commits already satisfy most of the requirements of this issue, though with a few rough spots:

• They use the elm-markdown package, which is based on marked instead of markdown-it. This sidesteps a few technical hurdles, but isn't the intended solution.
• There is no distinction between editing and simply viewing. The editor and Save and Delete buttons are always visible, and further, clicking Save or Delete offers no visual indication that anything has happened.
• Opening a nonexistent topic at first yields an ugly error, but this vanishes upon edit.
• Scrolling, links, and the back and forward buttons make a best effort, but still interact curiously. In particular, because content is persistently opened or closed, moving through history can put you at incorrect scroll positions. Further, the back button can reopen closed topics, but only if they were opened just prior, and the Close button surprisingly wipes out anything accessible via forward.

Regardless, the front end is in good enough shape to close this issue. The above can be more effectively tracked separately.