* scope fix: when using 'Custom Page Prefix scope (fixes#873)
- don't include primary seed URL in include list
- don't always add trailing slash to extra in scope URLs
- set seed scope to 'prefix' (supported via webrecorder/browsertrix-crawler#318) instead of re-including seed URL
- add comments on using 'custom' to indicate 'Custom Prefix Scope' semantics on frontend, setting actual scope to 'prefix' on backend
- remove unneeded conditional for additional urls, main scopeType overridden per seed anyway
ec3404c798