_content/blog: mark Go code blocks as Go

Label every Go code block in the blog with the "go" language,
in preparation for client-side coloring of Go comments and
string literals, which will key on these labels:

- Indented code blocks containing Go are converted to fenced
  ```go blocks (or ~~~go within {{raw `...`}} template strings,
  where a backquote would end the string).
- Fences labeled "Go" are changed to "go", for consistency and
  so that a case-sensitive class selector (language-go) matches.
- Raw <pre> elements containing Go (in gofix.md and inliner.md)
  get lang=go.

Blocks were classified using go/scanner-based heuristics
(snippets are often fragments, so go/parser was unsuitable),
then reviewed by hand. Shell sessions, tool output, C code,
protobuf, JSON, and the math notation of type-inference.md
are left unlabeled.

turkey-doodle.md's <pre> is deliberately left unlabeled,
since it contains hyperlinks that a token colorizer that
replaces the element's content would discard.

The rendered HTML of every affected post is unchanged apart
from the added class/lang attributes.

Change-Id: I3696d88c55ba80205417dcfbff554d2153486612
Reviewed-on: https://go-review.googlesource.com/c/website/+/842285
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Austin Clements <austin@google.com>