website: ability to add shortlinks

[dmitshur]

https://golang.org/s currently says:

Link creation temporarily unavailable. See golang.org/issue/27205.

golang.org/issue/27205 is closed now, but it had the following follow-up task that wasn't yet complete:

• ability to add shortlinks (old app had a login, GAE Flex doesn't have authed handlers)

This issue is now tracking that task.

[gopherbot]

Change https://golang.org/cl/160377 mentions this issue: godoc/short: point to new tracking issue for shortlink creation

[gopherbot]

Change https://golang.org/cl/160477 mentions this issue: [release-branch.go1.11] godoc/short: point to new tracking issue for shortlink creation

[gopherbot]

Change https://golang.org/cl/238545 mentions this issue: all: add cmd/admingolangorg