Add Gravity PDF Metabox to Gravity Flow Inbox

[jakejackson1]

Description

Add capability check before rendering the meta box in Entry List, Details, and Flow.

Testing instructions

1. Install Gravity Flow and create a User Input or Approval step
2. Setup multiple PDFs on the associated form - active, inactive, and conditional
3. Verify the meta box displays in the admin Inbox and front-end Inbox as the admin user

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: 15 lines in your changes are missing coverage. Please review.

Comparison is base (924f210) 75.13% compared to head (a351d25) 75.17%.

Files	Patch %	Lines
src/Model/Model_PDF.php	82.85%	12 Missing :warning:
src/bootstrap.php	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #1494 +/- ## ================================================= + Coverage 75.13% 75.17% +0.04% - Complexity 2953 2963 +10 ================================================= Files 243 243 Lines 12883 12941 +58 Branches 370 370 ================================================= + Hits 9679 9729 +50 - Misses 3196 3204 +8 Partials 8 8 ```

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