Add Txt Annotator

[JSv4]

Substantial amount of work to achieve the following:

1. Frontend support for labelling txt-based formats.
2. Backend support to trigger different processing pipelines based on the applicable file type.

Fixes #212

[codecov]

Codecov Report

Attention: Patch coverage is 95.52573% with 20 lines in your changes missing coverage. Please review.

Project coverage is 86.44%. Comparing base (71630c4) to head (b17c3d6).
Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
opencontractserver/shared/decorators.py	65.21%	8 Missing ⚠️
opencontractserver/tasks/doc_tasks.py	89.79%	5 Missing ⚠️
opencontractserver/utils/files.py	92.68%	3 Missing ⚠️
opencontractserver/tests/test_document_uploads.py	96.66%	2 Missing ⚠️
opencontractserver/tests/test_plaintext_checker.py	98.03%	1 Missing ⚠️
opencontractserver/tests/test_text_thumbnails.py	97.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #233      +/-   ##
==========================================
- Coverage   86.86%   86.44%   -0.42%     
==========================================
  Files         189      198       +9     
  Lines        6227     6449     +222     
==========================================
+ Hits         5409     5575     +166     
- Misses        818      874      +56     

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