Template cache flushes and performance improvements

[jakejackson1]

Description

This fixes two bugs related to the flushing of the PDF template cache, and makes a small improvement to performance when reading files from disk:

• Bug: Ensure the template cache is correctly cleared when PDF Debug Mode is enabled
• Bug: Flush the template cache after installing new templates via the PDF Template Manager
• Housekeeping: Small improvement to performance when reading template and font files from disk

Testing instructions

Screenshots

Checklist:

• [x] I've tested the code.
• [x] My code is easy to read, follow, and understand
• [ ] My code follows the accessibility standards.
• [x] My code has proper inline documentation / docblocks.

Additional Comments

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 76.96%. Comparing base (16173d7) to head (230e99b).

Files	Patch %	Lines
src/Helper/Helper_Templates.php	91.66%	1 Missing :warning:
src/Model/Model_Templates.php	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #1514 +/- ## =============================================== - Coverage 76.96% 76.96% -0.01% =============================================== Files 244 244 Lines 12742 12739 -3 Branches 370 370 =============================================== - Hits 9807 9804 -3 Misses 2927 2927 Partials 8 8 ```

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