Rewrite Filterable trait used for Squad filters

[recursivetree]

Problem:

@bluedagger on discord noticed that if a character is not in any alliance, and the squad contains a filter for alliance is not x, the user doesn't get added to the squad, even though he should.

How it has been fixed

While studying the Filterable trait code (https://github.com/eveseat/web/blob/c97b7cd1dd8cd5c063415ba0710d4b343dfdfef5/src/Models/Filterable.php), I noticed how messy and broken it was. I really struggled to first pin down the problem, and fixing it was even harder since it would have required to special case is not in multiple cases.

Therefore, I decided it's rewrite time! This allowed me to fix the original bug, and additionally:

• Make it more readable: There is no 8 levels deep nested code anymore
• Allow nesting of groups to arbitrary depths
• Add a new test case covering the bug

All squad tests are passing.

Lastly, I added a few files to the gitignore that were generated while running tests, but don't belong on github.

[recursivetree]

One difference to the old implementation is that we don't sort by relation. I don't quite understand why this has been done, and it also works without