The Go template that produces the search index has been overhauled in order to
handle the special case of "absract-only" pages. These are still classified as
"Essay" type but they have no content, only `abstract` frontmatter data.

The desired behavior for search is that on these pages, the user should be taken
to the section-head page that this abstract appears in if they choose a result
of this type.

In Go, the different cases (normal vs abstract-only pages) are handled via
conditional assignment of block-scoped variables in the local .Scratch (as
opposed to the page-level $.Scratch).

The additional logic comes into play if the length of a given page's .Content is
0 (i.e. empty). In that case, the abstract content is used instead, and instead
of using the normal .RelPermalink for the page, the URL data is set as the
relref of the page's parent section (figured out with a crude hack for now -
taking the name of the $page.Section value and adding "/index.md" on the end).

This removes some of Hugo's auto-generated pages for things like "Categories",
"Tags", etc. which are not being used in this project.

This needs a pass for QA as well as UI, but the basic functionality is now
working. Previous projects set this feature as something available on all pages;
in this book the user needs to navigate to the /search page to access this
screen. This simplifies things both in terms of CSS layout, but also performance
issues (the search index is 1MB, and now it doesn't need to be made available on
all pages, just the search-specific page).

A "search.md" page has been added to the contents folder. Any content here
will not show up, but we need to tell Hugo to build a page using the search
layout at this URL.

The global `baseof.html` template has been modified so that the search link now
appears in all of them (not the search button and results partial from before).
A new search/single.html layout has been added to the base theme which handles
this content instead.

Finally, the main UI.js file has been re-worked a bit to accommodate the new
behavior.

Rebuilds the search feature as a stand-alone Vue.js component for improved UX.
The new version of this feature supports the following:

* "Loading..." message until the search data is loaded
* Debounced input for the search query
* Getting user input, displaying results, etc. is much clearer in the code
  thanks to Vue's api (as opposed to doing everything manually with jQuery).

Adding the `online: false` parameter removes the page from the linear
reading order of the publication. This commit also hides the previous
and next arrows in the upper navigation bar, to make the search page
more of a standalone.