[Guilds] Allow guild onboarding without linking a Discord server

[alalonde]

What would you like to be added?

The guild onboarding flow was originally implemented primarily to let guilds define "membership" by certain roles in their Discord server. Currently we have a flag membership_through_discord on a guild which is always set to true for guilds going through this process.

However, guilds may want their memberships to be defined some other way; say, by those belonging to a DAO.

Why is this needed?

The end goal is to get as many guilds (esp. value-aligned guilds) into the system as possible. Most guilds have Discord servers but not all, and having that as a hard requirement seems unnecessary.

To implement this, we would need to change the /join/guild page to have two buttons, e.g. Apply with Discord and Apply to Join, with some text describing what it means to do either. This would set the membership_through_discord flag for the new guild (which should not be editable). The discord-specific guild fields would not appear for non-Discord applications.

A follow-up task for this would be to also automatically determine guild "membership" through a linked DAOHaus DAO. This should probably have its own issue.