Switch from tipue to lunr for search

[d7919]

The tipue search plugin is not actively maintained and hence we are reliant on a frozen version of the package shipped with Ford. This PR switches to lunr.js. This particular search option was simply motivated by the existence of some examples and good compatibility with the existing tipuesearch infrastructure. For example https://learn.cloudcannon.com/jekyll/jekyll-search-using-lunr-js/ and https://github.com/Pelican-Elegant/elegant/pull/573/files .

Pros:

• Use of an actively maintained plugin
• No explicit dependence on jQuery
• Can be included via a CDN, reducing storage / ship size of website with search.

Cons:

• Include via a CDN, search won't work on a machine with no internet connection.
• May be slower than tipue search (no measurements, just feel)
• Less pleasing out of the box styling of search results (possible we can customise css for this)