remove duplicate code for tags and categories

Previously the `Site#tags` and `Site#categories` actually had the
same structure as a hash returned by `Site#post_attr_hash()`, but
maintained separately. Remove this duplicated infrastructure. For
backwards-compatibility for plugins, provide aliases for them.
Merge pull request #1639 from maul-esel/tags-and-categories
Update history to reflect merge of #1639 [ci skip]
Update site_template/_includes/head.html.

* remove `chrome` from `X-UA-Compatible`; it's been discontinued
* remove unneeded end tags
* remove unneeded slash in `canonical` link
Merge pull request #2186 from XhmikosR/site_template
Update history to reflect merge of #2186 [ci skip]
docs: remove unused images.
Merge pull request #2187 from XhmikosR/rm-unused-img
Update history to reflect merge of #2187 [ci skip]
Move vendor prefixed properties before the unprefixed ones.
Use the shorthand font property when possible.
Be explicit with the `background` property.
Remove units from zero.
Use `font-weight: 700` consistently.
Remove duplicate properties.
Add fallback colors.
Add missing vendor prefixed properties.
Simplify selectors.