Simplify interlinear model (#63)

* Simplify interlinear model: remove InterlinearAlignment/InterlinearText, add ActiveProject

* Fix model gaps for lossless LCM / PT9 / BT Extension import

* Make `analysisLanguages` required

* Add comments about mapping of BT Extension's `sideNum`

* Update docs/schema

* Further refinement; please see updated description

* Suggested model tweak

* Model idea: Split linking out from analyses

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Co-authored-by: D. Ror. <imnasnainaec@gmail.com>

Author: alex-rawlings-yyc