[Homebrew] Allow All Spells in Public Homebrew

Currently when creating homebrew, adding a spell outside of the Basic Rules or Elemental Evil Player's Companion makes homebrew items, sub-classes etc. ineligible for sharing with the community.

I would like to propose that homebrew should still be shareable with the following changes:

• When a user attempts to add the item/race/sub-class etc. they will receive a warning if any of the spells for a feature belong to a book that they do not have unlocked, and so will not appear. Ideally there should be a list of which spell(s) are missing, with options to buy them (or the book they are from).
• In Homebrew searches, add a Source box as found in many of the non-homebrew sections (Monsters, Spells etc.), enabling users to limit homebrew searches only to sources that they wish.

For example, let's say I add the spell Chaos Bolt to my awesome new sorcerer sub-class as part of its extra spell list, since this spell comes from Xanathar's Guide to Everything this will currently limit my sub-class to private use only. But some users might enjoy my sub-class enough that they would like to use it even with the missing spell.

Optionally, missing spells could be replaced with choices, in the same way as an empty spell entry (which allows a player to choose a spell to add, such as for Magical Secrets on a Bard). This way while a character wouldnt have the spell I intended, they wouldn't be disadvantaged by not having it as they could pick something else themselves.