delicious digests

This patch changes the publish URL from the current
```
https://lolcat.makes.org/thimble/a-thing-I-like
```
to
```
https://lolcat.makes.org/thimble/Hj67Wh5EU/a-thing-I-like
```
where the url is "domain" + "tool" + "project id digest" + "vanity title". The vanity title doesn't really do much but it helps users identify content.

The patch has also been written to allow backward compatibility:
- new projects use the new URL scheme
- old projects use the old URL scheme
- remixing an old project uses the new scheme (due to being a new project)
- editing an old project uses the old scheme (so as not to break existing links on the interwebs)

There are two things worth thinking about here:

1. the project ID is currently a 32 bit integer. This leads to a fairly long project id digest, while only (heh, "only") allowing about 4 billion makes. We might be able to come up with a better scheme before landing, although the digest should not be a hash, as we don't want the headache of resolving hash collisions.
2. if people edit their "old style" project, and retitle it, should this still use the old scheme, or can we treat it as a republish, thus letting us use the new scheme?