Replace mPDF HTTP Request with wp_remote_get()

[jakejackson1]

Description

Resolve issues loading images / assets when using mPDF's native cURL functionality.

Fixes #1440

Testing instructions

Screenshots

Checklist:

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

Additional Comments

[codecov]

Codecov Report

Merging #1441 (157701d) into development (b9671fd) will increase coverage by 17.60%.
The diff coverage is n/a.

@@                Coverage Diff                 @@
##             development    #1441       +/-   ##
==================================================
+ Coverage          75.70%   93.30%   +17.60%     
==================================================
  Files                243       57      -186     
  Lines              12826     1464    -11362     
  Branches             370      370               
==================================================
- Hits                9710     1366     -8344     
+ Misses              3108       90     -3018     
  Partials               8        8               

Impacted Files	Coverage Δ
src/Helper/Helper_PDF.php
src/Helper/Fields/Field_Tax.php
src/View/html/Settings/tools.php
src/Helper/Helper_Options_Fields.php
src/templates/zadani.php
src/Model/Model_Mergetags.php
src/Helper/Fields/Field_Quantity.php
src/Model/Model_PDF.php
src/Controller/Controller_Settings.php
src/Controller/Controller_Install.php
... and 176 more

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