offline functionality

[mikonse]

currently the web app has no offline capabilities at all and does not deal well with unstable / slow connections as updates to local data is only fetched after websocket notifications.

Measures to decouple the websocket notifications from data changes due to actions:

• [x] after an action return the new model state from the db, e.g. after changing a transaction share return the new (or partial) transaction in the post response. Determine if it smarter (or easier) to only send partial updates to the model or always send the full model
• [x] use websocket notifications only to load changes other users (sessions of the same user) made

Further improvements for future offline capabilities

• [x] implement API endpoint with which to fully sync a transaction state (including metadata as well as shares)
• [x] implement API endpoint with which to fully sync a transaction position (including metadata as well as position shares)

[mikonse]

postponed for now, groundwork is done in the app, but due to too much complexity will not tackle