Use functions, instead of R Markdown components, to render value boxes

[nsunami]

Currently, the app is rendering value boxes from another R Markdown file, using the chunk options (child=). I have a felling that this way of handling reused components will not be sustainable, especially when considering converting this app to a package.

Instead, I'd suggest using functions to render value boxes. We need to do this for all the value boxes that we have.

[nsunami]

Just a note that resolving this issue fixes #52 since we won't be reading external markdown files (and thus no title metadata will be needed for each)