Inherit diary from aircon-header

This face seems to be used for the date at the beginning of a line in
diary-mode (M-x diary-show-all-entries).
Inherit diff-error from error
Remove redundant provide
Style gnus-group-mail-1-empty with hibiscus
Add customization group and prepare for GNU ELPA

Thanks to Philip Kaludercic for these suggestions.
Keep foreground colours in magit-section-highlight
Apply mango to certain Magit heading selections

These faces appear when you select e.g. untracked files, diff
headings, log headings.

It might be too strong to use mango as a selection face here,
especially when the normal ‘region’ face is a subtle grey. However,
the subtle grey background occurs next to the slighly stronger bluish
grey of the ‘highlight’ face used in some Magit contexts, which does
not communicate well that it is an active region/selection to my
eyes. This is probably a wider problem with the ‘highlight’ and
‘region’ faces in general, but it's especially problematic here. So
I've had to resort to mango -- at least it's cohesive with marked
files in dired.
Style gnus-summary-cancelled

Used for e.g. moved article
Specify min-colors 89 for aircon-header
Style Magit headers and highlights
Refactor scorpion face to be derived from const
Label some colours with semantic intention
Embolden gnus-summary-normal-ticked (sapphire)
Style dired-special with aircon-hibiscus
Style dired-perm-write with aircon-goblin
Inherit more faces from error, warning, success
Inherit font-lock-warning-face from warning
Style elisp-shorthand-font-lock-face
Style EWW certificate faces (-valid-, -invalid-)

These affect the appearance of the page title in the header of EWW