CI: check external links via cron build

[chalin]

• [x] Setup cron job.
• [ ] Adapt scripts so that external link checking is done only in a cron job.

[chalin]

It might be possible to fix #1107 at the same time.

[kwalrath]

We should add a test that looks for bad external links, but (unlike flutter.dev’s current behavior) I don’t think we should fail the build just because an external URL no longer works.

Related: #1685, https://github.com/flutter/website/issues/2734

[chalin]

Since this issue has been opened, linkcheck has been updated, and no reports command failures on connection failures. That makes external checking much more robust.

Note that most of the machinery is in place for this, and link checking (which is currently only for internal links), is allowed to fail w/o causing Travis to be alarmed. So, if we don't mind doing external link check for every PR and master build, then all we need to do is use the --external flag of check-links.

Otherwise, a little bit of config could ensure that we only do external link checking for, say, daily cron job builds.

WDYT?

[chalin]

For the Flutter website it was suggested that we only check external links every X days, see https://github.com/flutter/website/issues/3079. I assume that we'll want something similar here too?

[chalin]

Cron job scheduled to run daily:

First cron job run successfully (and none of the jobs trigger a site deploy, per #2011 -- also related is https://github.com/flutter/website/issues/3127):

cc @johnpryan @legalcodes