Optimise download many

[ObadaS]

Original PR #1999 from @nicomy

@ mention of reviewers

@Didayolo @ObadaS @ihsaan-ullah

A brief description of the purpose of the changes contained in this PR.

This PR improves the Bulk Download functionality.

The preivous bulk download triggered multiple database queries, which was inefficient and error-prone. This PR refactors the process so that:

• A single server-side query is executed.
• The server generates download URLs and sends them to the client.
• The client downloads files and builds a zip archive containing all submissions.

A progress bar UI try to give users feedback on the download progress.

Issues this PR resolves

• Reduced database load by consolidating queries into one.
• Avoided previous errors caused by multiple queries during bulk download.
• Improved user experience with a visible progress indicator.

Related:

• "Download selected submissions" not working #1551

A checklist for hand testing

• Verify that the updated view renders correctly and the layout is visually pleasing.
• Trigger a bulk download on the test database and confirm that previous errors no longer occur.
• Confirm that the progress bar appears only when the download starts and updates during the process.
• Ensure that the zip file contains all expected submissions.

Any relevant files for testing

Previous Issue

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge

[Didayolo]

Flake8 issues to be fixed:

^@^@src/apps/api/views/submissions.py:221:1: W293 blank line contains whitespace
src/apps/api/views/submissions.py:223:5: E303 too many blank lines (2)
src/apps/api/views/submissions.py:223:42: E231 missing whitespace after ','
src/apps/api/views/submissions.py:223:50: E231 missing whitespace after ','
src/apps/api/views/submissions.py:416:5: E265 block comment should start with '# '
src/apps/api/views/submissions.py:416:174: W291 trailing whitespace
src/apps/api/views/submissions.py:418:5: F811 redefinition of unused 'download_many' from line 407
src/apps/api/views/submissions.py:426:1: W293 blank line contains whitespace
src/apps/api/views/submissions.py:471:35: E201 whitespace after '['
src/apps/api/views/submissions.py:471:57: E261 at least two spaces before inline comment
src/apps/api/views/submissions.py:471:58: E262 inline comment should start with '# '
src/apps/api/views/submissions.py:475:26: F821 undefined name 'serializer'
src/apps/api/views/submissions.py:476:13: E265 block comment should start with '# '
src/apps/api/views/submissions.py:476:136: W291 trailing whitespace
src/apps/api/views/submissions.py:479:1: W293 blank line contains whitespace
src/apps/api/views/submissions.py:481:1: W293 blank line contains whitespace