feat(FastAPI): add pkgbase comments (new, edit)

In PHP, this was implemented using an /rpc type 'get-comment-form'.
With FastAPI, we've decided to reorganize this into a non-RPC route:
`/pkgbase/{name}/comments/{id}/form`, rendered via the new
`templates/partials/packages/comment_form.html` template.

When the comment_form.html template is provided a `comment` object,
it will produce an edit comment form. Otherwise, it will produce a new
comment form.

A few new FastAPI routes have been introduced:

- GET `/pkgbase/{name}/comments/{id}/form`
    - Produces a JSON response based on {"form": "<form_markup>"}.
- POST `/pkgbase/{name}/comments'
    - Creates a new comment.
- POST `/pkgbase/{name}/comments/{id}`
    - Edits an existing comment.

In addition, some Javascript has been modified for our new routes.

Signed-off-by: Kevin Morris <kevr@0cost.org>