Refactor graphImageResponse to use Temporary Files

[a1-su]

Proposed Changes

This PR refactors the graphImageResponse function in Controllers/Graph.hs to use withSystemTempFile temporary files (from the temporary library in Haskell). Previously the files were being manually generated and deleted, so if an error ever occurred during process of saving the graph, the files would be left undeleted.

Note that this modified some other functions, including getActiveGraphImage in Export/GetImages.hs, which was coupled with the exportTimetablePDF function in Controllers/Timetable.hs. I've modified getActiveGraphImage to keep its original functionality and graphImageResponse to use helper function writeActiveGraphImage, but should exportTimetablePDF ever get refactored to also use temporary files and writeActiveGraphImage, getActiveGraphImage can be safely removed.

Type of Change

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Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)
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🎨 User interface change (change to user interface; provide screenshots)
♻️ Refactoring (internal change to codebase, without changing functionality)	X
🚦 Test update (change that only adds or modifies tests)
📦 Dependency update (change that updates a dependency)
🔧 Internal (change that only affects developers or continuous integration)

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[coveralls]

Pull Request Test Coverage Report for Build da278d0c-dde3-470e-b0ed-e13fa305e2f9

Details

• 0 of 23 (0.0%) changed or added relevant lines in 3 files are covered.
• 30 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 56.307%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
app/Controllers/Graph.hs	0	6	0.0%
app/Util/Helpers.hs	0	6	0.0%
app/Export/GetImages.hs	0	11	0.0%

Files with Coverage Reduction	New Missed Lines	%
app/Export/GetImages.hs	30	0.0%

Totals
Change from base Build e318a04a-7db4-4eb7-98e7-21e1ce93c3b7:	-0.1%
Covered Lines:	2280
Relevant Lines:	3977

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💛 - Coveralls