HasCustomFields Trait - rebased

[uberbrady]

This is a squashed branch of all of the various commits that make up the new HasCustomFields trait.

This should allow us to add custom fields to just about anything we want to within Snipe-IT.

Below are the commits that have been squashed together:

Initial decoupling of custom field behavior from Assets for re-use

Add new DB columns to Custom Fields and fieldsets for 'type'

WIP: trying to figure out UI for custom fields for things other than Assets, find problematic places

Real progress towards getting to where this stuff might actually work...

Fix the table-name determining code for Custom Fields

Getting it closer to where Assets at least work

Rename the trait to it's new, even better name

Solid progress on the new Trait!

WIP: HasCustomFields, still working some stuff out

Got some basics working; creating custom fields and stuff

HasCustomFields now validates and saves

Starting to yank the other boilerplate code as things start to work (!)

Got the start of defaultValuesForCustomField() working

More progress (squash me!)

Add migrations for default_values_for_custom_fields table

WIP: more towards hasCustomFields trait

Progress cleaning up the PR, fixing FIXME's

New, passing HasCustomFieldsTrait test!

Fix date formatter helper for custom fields

Fixed more FIXME's

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context, providing screenshots where practical. List any dependencies that are required for this change.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• 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