feat: drop mptt dependency

Warning

This PR is for discussion. It is a proof of concept that filer can work without mptt (or any other tree library). Please do comment, but do not merge.

Description

Trees stored in databases are a pain since they can generate many database hits ("n+1" issue). django-mptt circumvents this by getting full horizontal cuts through a tree in one go, trading database hits for superfluous data that needs to be transferred and processed. To be able to do this, mptt keeps extra data for each folder (which when updating might cause race conditions).

For filer, I propose not using any tree libraries (mptt, treebeard or other), since the benefits are small

File trees are typically not deep ("small n"), say < 10 levels.
They can be very wide (many files in one folder)
Only few operations require traversing the tree descendants:
- Setting rights recursively
- Deleting a folder (and thereby all its content)
Only getting the "full path" requires traversing the ascendant. (Which is not optimized by mptt btw.)

Many day to day actions such as serving files, moving folders, etc. are inherently efficient operations.

Evaluation

This discussion PR makes the case for just dropping the mptt dependency. It speaks for itself how little changes are necessary to implement it.

Please use it for tests and performance evaluation.

The test suite does not contain performance tests. Maybe we can change this?

My performance hypothesis:

n+1 issues will be negligible, even improve performance (less data in more db hits)
performance is muddied by easythumbnail's "m+1" issue in checking the database for the availability of thumbnails. (Here is see separate performance improvements.)
Not needing to update the whole tree when, e.g., moving a folder increases consistency and performance. This alone removes an "n+1" issue when moving objects

Performance

Manual inspection with the debug toolbar shows that dropping mptt leaves the number of queries per folder viewed unchanged.

Related resources

1348
This PR contains a stub mptt/__init__.py since app-helper forces the test runner to have mptt in its INSTALLED_APPS

Checklist

[x] I have opened this pull request against master
[x] I have added or modified the tests when changing logic (N/A - same logic just no mptt)
[x] I have followed the conventional commits guidelines to add meaningful information into the changelog
[x] I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.

Codecov Report

Merging #1351 (5126eca) into master (34803bc) will increase coverage by 0.06%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1351      +/-   ##
==========================================
+ Coverage   72.39%   72.46%   +0.06%     
==========================================
  Files          72       73       +1     
  Lines        3268     3258      -10     
  Branches      532      530       -2     
==========================================
- Hits         2366     2361       -5     
+ Misses        735      731       -4     
+ Partials      167      166       -1

Impacted Files	Coverage Δ
filer/admin/folderadmin.py	`72.14% <100.00%> (+0.27%)`	:arrow_up:
...der_index_together_remove_folder_level_and_more.py	`100.00% <100.00%> (ø)`
filer/models/filemodels.py	`86.48% <100.00%> (-0.07%)`	:arrow_down:
filer/models/foldermodels.py	`86.86% <100.00%> (+0.39%)`	:arrow_up:

django-cms / django-filer