PR#2355 Continued + expanded scope (#2365)

* #2317 Fetch pinned documents once per folder to reduce the number of queries.

* Reorder the lines to keeps const declarations together.

* Add some comments to functions
move pinned document fetch for folder to function
move watched documents per-folder to also function the same
remove unused function in documents model

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Co-authored-by: Błażej Owczarczyk <blazeyy@gmail.com>