Fixes Customfields with boolean format not saving [ch-9973]

[inietov]

Description

I change the value of the CustomField with boolean format if gets as string instead of boolean type, because if a string with "true", "false", "1" or "0" values doesn't pass the boolean validation.

I also change the 'show' and 'edit' views for this format to look like a green check if true and a red 'x' if false.

Fixes clubhouse story 9973

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Test Configuration:

• PHP version: 7.4.16
• MySQL version: 8.0.23
• Webserver version: nginx/1.19.8
• OS version: Debian 10

Checklist:

• I have read the Contributing documentation available here: https://snipe-it.readme.io/docs/contributing-overview
• I have formatted this PR according to the project guidelines: https://snipe-it.readme.io/docs/contributing-overview#pull-request-guidelines
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes