Add support for importing / exporting dashboards, stories and contexts

[MV88]

Description

let's add import/export tool also for other resources different from the Map

Acceptance criteria

Add in the related burger menu the possibility to

• [x] import a dashboard (from json)
• [x] export a dashboard (to json)
• [x] import a geostory (from json)
• [x] export a geostory (to json)
• [ ] import a context (from json)
• [ ] export a context (to json)

Other useful information