New architecture; stop using Elasticsearch

[osma]

This is quite a drastic change from before, but I think it's the right thing to do to get things going.

Drop the use of Elasticsearch (currently used only to manage projects) and switch to a simpler architecture where projects are defined in a configuration file (projects.cfg).

This also means that we can drop the project management CLI and REST operations. Projects can be read-only.

The index functionality can be later implemented better using gensim and/or fastText. These will become backends and there can eventually be several backends per project, as defined in another configuration file (backends.cfg).

[osma]

Need also to update doc/architecture.md