Maui Server backend, including training support

[osma]

The Maui Server API provides all the important Maui functionality over REST. Annif should have a backend that works with this - either by enhancing the http backend or by creating a new maui backend. The latter is probably easier, though even the http backend could be quite easily made to support at least the basic analyze/suggest case just by making the response handling more robust so it will accept Maui Server responses as well.

All the configuration for the Maui Server tagger should go in the Annif projects.cfg configuration. There could be a new Annif command init (#251) that creates the tagger on the Maui server end, based on the configuration. Possibly it would upload the vocabulary to Maui as well, though then loadvoc needs to be executed first. The other (probably better) option would be to add a hook for notifying backends about loading the vocabulary, which would trigger the upload.

[osma]

A new version of Maui Server that includes the NatLibFi enhancements to Maui (e.g. support for Finnish and Swedish) is being developed at https://github.com/NatLibFi/MauiServer/

We need to start working on this backend soon.

[osma]

One challenge is that Maui requires a vocabulary in SKOS format. Annif does support loading a SKOS vocabulary, but currently it doesn't store the SKOS file at all, it just extracts URIs and prefLabels and saves these into a TSV file.