Support multiple named custom profiles, allow overriding built-ins, and picking a default

[hhkaos]

`ote.config.json`'s `customProfile.fields` (per the editor README) supports exactly one custom profile today, layered on top of the three built-ins (`meetup`, `conference`, `all`).

Requested:

• Define several custom profiles, each with its own name (not just the single implicit `customProfile`).
• Let a custom profile override a built-in one by reusing its name (`meetup`, `conference`, `all`), instead of only being able to add new ones alongside them.
• Choose which profile (built-in or custom) is the default shown when the editor opens for that repo, instead of always defaulting to `all`/`meetup` per the current logic.