Evonne Cheung authored Jul 21, 2020 and Robert Lyon committed Sep 17, 2020

Base font has been increased to 16px, and as a result, variables
have had to be recalculated.

Max browser width has been increased to 1440px to allow the
larger font to have more characters in a line.

As a part of accessibility, headings H1-H6 have had to be rejigged.
Note that for elements such as popup/modals, the heading restarts
at H1 again as advised by Julius.

Page heading is H1, block heading is H2, everything else should be
H3. Trying to attempt to reserve H4-H6 for user content, but it is
still existing in some places as some things can't be changed.

Attachment markups have been unified and added to styleguide.

Removed H3/H4 from fieldset legends.

behatnotneeded

Change-Id: I69273c9775d658f03738c29be8d230b59c189790