Update to Bootstrap 3.4.1 and jQuery 3.4.1

[StevenMaude]

Following getpelican/pelican-themes#659 and getpelican/pelican-themes#660, issue for my usage is that I need to make some tweaks to the Bootswatch theme.

Notes I have from previously:

## Theme customiser
DONE:
Cosmo
Change font size to 22px
Change fonts to Source Sans and Source Code
Set line height to 1.454545454
Pagination button colours: pagination-color rgb(50, 50, 50)
pagination-hover-color: rgb(0, 0, 0)
In theme.less:
@import url("https://fonts.googleapis.com/css?family=Source+Sans+Pro:300,400,700");
Change code font to "Liberation Mono", Cousine, Consolas
Replace bootstrap-live-customizer URL with .. (Glyphicons)