Update jQuery from v1.12.4 to v3.5.1

[chao-xian]

jQuery v3.5.1. available from:
https://code.jquery.com/jquery-3.5.1.min.js

[ESKYoung]

Thanks for raising the draft PR @chao-xian! I've made an attempt at updating third-party JavaScript files. jQuery and Lodash are now up-to-date. Other points to note:

1. (lib/assets/javascripts/_vendor/modernizr.js) The existing Modernizr v3.3.1 file appears to be a custom build (see line 2). However, the link in line 2 to the Modernizr site should not be used, as the site is out-of-date and broken
   • I tried rebuilding v3.3.1 using the same build parameters, but there appeared to be differences with our file
   • The latest version is v3.11.4, but I'm not sure how to make the custom build or if I should given the above
2. (spec/javascripts/helpers/lunr.min.js) I tried updating Lunr to v2.3.9, but the tests failed, and the built application.js file still had v0.7.0 in it, so I rolled back those changes
3. (lib/assets/javascripts/_vendor/jquery.mark.js) mark.js seems to be latest release, but the repo seems to be stale (last master commit in Aug 2019)
4. (lib/assets/javascripts/_vendor/fixedsticky.js) Latest release of fixedsticky.js, but this repo has been deprecated.

[lfdebrux]

Re-opening this so we can rebase, review and merge it

[lfdebrux]

Thanks for the contribution @ESKYoung , @chao-xian. I especially appreciate the detailed comment explaining the work done.

Sorry it took so long for us to get around to reviewing it, the tech-docs-gem has been languishing for a bit, but we have a new working group now! 🎉

Hopefully I should be able to get this merged ASAP.

[lfdebrux]

This looks good to me; I've reviewed the changes in lodash.js, confirmed that jquery.js matches what is on code.jquery.com, and checked that the search still works when using the gem in the GOV.UK Frontend tech docs.

I think this just needs an approval from @m-green, and it can then be merged.

[m-green]

Happy to approve from my side - thank you everyone!

[chao-xian]

Thanks everyone!