Fixes slack settings submit button availability

[Godmartinz]

Description

This greys out the submit button if slack_endpoint or slack_channel are not filled in. Also greys it out if the integration
test fails or hasn't been run.
Fixes #SC-20041

Type of change

Please delete options that are not relevant.

• [ x] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Test A
• Test B

Test Configuration:

• PHP version:
• MySQL version
• Webserver version
• OS version

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

[what-the-diff]

• Added a new variable $isDisabled to the class SlackSettingsForm.php
• Updated function render() in SlackSettingsForm.php, added if statement that checks whether slack_channel and slack_endpoint are empty or not, then sets isDisabled accordingly
• Updated function testSlack(), set isDisabled='' when success message appears
• Changed button type submit from disabled to {{$isdisabled}} in view file slack-settings-form blade php