Remove imports/get and use api.js to avoid a race condition

The process status page loaded a process twice due to calls to both process/retrieveProcess and imports/get. state.imports.dataimport was already no longer used, so I removed the state, mutation and action, and removed useless commits from other methods. I added a few endpoints in js/api.js to also ensure workflows and tasks do not create duplicate requests either, even with the well-known "blinking status page" bug. After doing those changes, I could no longer reproduce a situation in which a status page never loads.

Closes #402 (closed)