Fix #1810 -- use watchdog in auto

[Kwpolska]

This is #1810.

Fixes (some of which would be useful even without watchdog):

• make rebuilds actually work cherry-picked onto master
• rebuild only once by using a set
• tell people we’re rebuilding and show them the log when we’re done

Also, we have to watch the directory with conf.py and filter out events because you must watch entire directories with watchdog.

cc @ralsina

[ralsina]

You mean rebuilds were not working?

[Kwpolska]

yeah, I broke them in f1e95ef and nobody really noticed…

[ralsina]

let's merge this then :)

[Kwpolska]

So is pyyaml no longer a huge problem?

[ralsina]

Well, at least cherrypick the fix? We can merge this later, then.

[Kwpolska]

done

[ralsina]

I'd watch all the theme folder, not theme/templates because of assets.

[Kwpolska]

Fixed in 6b43608.

[ralsina]

@Kwpolska let's merge this, dependencies be damned.