Add SimplenoteImporter

[mirka]

Depends on #922

• Is the class name good? Should it be called SimplenoteJSONImporter? 🤔
• This assumes that the import will be more or less instantaneous once the JSON file is read and parsed. (If this is insufficient, we might want to consider emitting the progress status events from CoreImporter instead of the source-specific parsers?)

[roundhill]

I have a Simplenote export file with 1,600 notes that is just over 2mb. Maybe we should set a 5mb limit? Not sure if you tested it with a large export or not.

[mirka]

Today I tested with export files of varying sizes (80KB – 4MB). While large files did not choke the app in terms of memory, it did degrade the experience in non-trivial ways:

1. There is considerable lag between the point when all notes have been added to the noteBucket, and the point when the notes appear (all at once) in the Note List.
2. Clicking on notes in the Note List is unresponsive for a while. The onClick is firing, but the redux dispatch is being deferred until a certain point... I'm not immediately sure what is causing this.
3. It takes quite a while for all the notes to be synced over to the server, and it is almost impossible to tell when it has finished. The only way to tell is to try clicking the Log Out button and see if it throws an unsynced notes warning.
4. If there are lots of notes AND lots of tags, the tags take a long time to show up in the tag drawer.

With these constraints, I felt like a 500-note (~400KB) export file was as far as I wanted to go... 😵 And since these issues are occurring after the CoreImporter is done, I'm guessing the other importers will encounter them too.

I'll see if there are some easy things we can do to address at least 1 and 2.

[mirka]

^ Addressed in #986 #987 #988

[roundhill]

• Is the class name good?

It WFM 👍

[roundhill]

This will need to be based on master now that we've merged the core importer.