Introduce item levels

[mhauru]

@nbarlowATI, I started working on adding a level field to all the case items. Restructuring models.py a bit in the process too, and trying to prepare the ground for more many-to-many parent fields and multiple types of parents. This is work in progress, and I'm making a PR draft just to have a place where to discuss this stuff. Feel free to push to this branch if you work on the same feature.

EDIT: No longer work in progress. This adds levels (only) to property claims, makes it possible for claims to have claims as parents, adds a frontend button for creating such subclaims, and reorganises some things in the process. Oh, and adds created_date to all items.

[coveralls]

Pull Request Test Coverage Report for Build 2097414184

• 40 of 49 (81.63%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-90.5%) to 0.0%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
eap_backend/eap_api/models.py	29	31	93.55%
eap_backend/eap_api/views.py	2	9	22.22%

Totals
Change from base Build 2078504785:	-90.5%
Covered Lines:	0
Relevant Lines:	0

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💛 - Coveralls

[nbarlowATI]

Excellent - even if we don't end up needing to use Levels, there are still some super-useful changes here - I like the idea of the CaseItem abstract base class.

[mhauru]

Ah yes, I already kinda forgot that we concluded that levels are probably unnecessary after all. I could make a PR removing them.