Generators and Permalinks

[sebb <se...@gmail.com>]

It's probably a good idea to extract the generator code into a separate file.

That will make it easier to test and maintain.

However it does not solve the fundamental issue which is that the
requirements for a database id are not the same as the requirements
for a Permalink id. Yet the output is being used for both.

Database id:
- needs to be unique within a database for all possible messages
otherwise some messages cannot be stored (this is intrinsic to
ElasticSearch)
- it does not need to be unique across databases
- it does not need to be preserved if the database has to be reloaded.

Permalink id:
- once a Permalink has been published, it must always be possible to
find the message
- if a mail archive has to be recreated, it must be possible to
generate the same id
- ideally, every id should be distinct
- ideally, every message should have an id.