Ensure project summary consistent on project saving/validation

[niklub]

In some cases (e.g. bulk removals of tasks/annotations) project summary is not updated correctly. This workaround helps to keep the project summary consistent with empty projects

[codecov]

Codecov Report

Merging #1178 (ab721e4) into master (007220c) will increase coverage by 3.60%.
The diff coverage is 89.12%.

@@            Coverage Diff             @@
##           master    #1178      +/-   ##
==========================================
+ Coverage   70.15%   73.75%   +3.60%     
==========================================
  Files         113      115       +2     
  Lines        7576     7720     +144     
==========================================
+ Hits         5315     5694     +379     
+ Misses       2261     2026     -235     

Impacted Files	Coverage Δ
label_studio/core/middleware.py	86.07% <ø> (-1.71%)	⬇️
label_studio/data_export/urls.py	100.00% <ø> (ø)
label_studio/data_import/urls.py	100.00% <ø> (ø)
label_studio/data_manager/api.py	77.63% <ø> (-0.14%)	⬇️
label_studio/data_manager/models.py	94.73% <ø> (ø)
label_studio/organizations/models.py	68.29% <ø> (ø)
label_studio/projects/urls.py	100.00% <ø> (ø)
label_studio/users/forms.py	89.47% <ø> (+10.80%)	⬆️
label_studio/organizations/functions.py	76.92% <50.00%> (-23.08%)	⬇️
label_studio/core/version.py	63.93% <61.90%> (+1.67%)	⬆️
... and 60 more

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