The need for this update was mainly to get type information for
`marked`; version 14.1.3 was current on the day when I made this
change. (The library releases and updates versions _frequently_.)

I did not carefully review the five years worth of changelogs between
0.7 and 14.1.3, but empirically the API doesn't seem to have changed
and the code still works.

Deletes old Pages Router blog files, as well as displayMarkdown
component.

…URLs [#134]

* Prepend `/blog/` to all blog post image URLs
* Remove `https://nextstrain.org` from link URLs