Toward clean/fork-friendly/contribution-friendly code

[robindemourat]

This is an on-going issue aimed at listing all chore/cleaning tasks remaining to make dicto code base effectively open source, that is easy to read, fork, and improve.

• add a CONTRIBUTE.md guide
• add a MANUAL_E2E_CHECKLIST.md guide

Stabilize style system

• replace redundant inline modification to css theme by css theme changes
• replace useless inline styles with css system

Clean react code

• refactor modals with a standard modal component
• refactor modal contents in separate components
• spot redundant html patterns and refactor them as reusable components
•  in features, refactor long layout component by externalizing feature-related components
• in features, move logic code toContainer components, dedicate Layout components layout/design
• drop inline callbacks in react components

Local code simplifications

• use lodash.get instead of multiple existence tests

Add unit tests to utils modules

Code documentation

• add esdoc header for all functions/classes

Resolve last performance bottlenecks

• composition edition view : do not load whole corpus data / handle data display in an async way (as for media edition)
• composition edition view : improve asides edition modal performance

Questions / to decide

Update to react 17 ?

This would mean refactoring everything relying on components livecycle.

Redux data management logic : share code between electron main process and renderer process ?

This would ask to find a way to keep an architecture which work both for webapp and electron app.