Refactor(NoteList): Reshape composite note list data structure

[dmsnell]

When adding tag-suggestions in the note list we create a "composite list" of notes and placeholders. The placeholders designate where headers will go, where the tag suggestions list will go, and where a message will go if there are no matching notes.

There were two issues with this structure:

• it was difficult to type because of how its structure overlapped a NoteEntity but didn't have a key field for discriminating the type. everything works just fine but the type signatures become harder to write.
• we were storing display information in the data property and that, as a render concern, wasn't necessary

Further, we were directly mutating notes which came from filterNotes() and the Redux state. We want to avoid directly mutating values we receive from other places in order to keep data flows clear and obvious.

In this patch we are replacing the object data type for placeholders with simpler string counterparts, moving the label text into the render function, and exchanging a mutating operation for a non-mutating one.

This is part of broader work to separate modifying the search parameters from
actually filtering the notes and was originally created as part of an
exploratory PR #1807.

Testing

This affects the tag suggestions auto-completer and the note list when searching.
Smoke test by searching in various ways: with filter text, with a tag from the tag list, with tag:something in the filter text, with the trash selected, with no notes matching the search, etc…

In early testing at one point I had the notes header and tag-suggestions header backwards, for example.

[belcherj]

Much cleaner!

[dmsnell]

Updated to rebase RELEASE-NOTES with recent changes from develop