[Feature] Add a "Custom characters" option to the relic potential view, for selecting a subset of all characters to score

[fribbels]

Motivation

Currently we only show All characters. Initially we also had an option for "Owned characters" but some users found that confusing. I think the best alternative is to have a separate view for "Custom characters" that selects all characters by default, and then users can choose which ones to "discard" from the subset.

Objective

Requires some cleanup first

• Refactor the character selector and light cone selector from the Optimizer tab into one component
• Currently they only show one option but enable a multiselect option for this component
• Add a new button for the relics tab to customize the subset of characters and open this mutli selector character model
• Have a new view under relic ratings to show Max potential Custom characters

[fribbels]

Released in #298