[SETTINGS] Allow editing corpus settings when no webentity in corpus yet

[boogheta]

The SETTINGS page should allow to edit some of its fields on the condition that the corpus has no webentity and no crawl yet:

• defaultStartpagesMode (with checkboxes for "homepage", "startpages", "pages-5", "prefixes"
• proxy host & port
• defaultCreationRule (with a radio input between "domain", "subdomain", "page")
• follow_redirects domains (remove or add some)
• better handle error cases (cf 18bfc59#r35430034 )
• do not cancel when invalid checks (cf 18bfc59#r35429981 )
• startpagesmode: allow to change the N of pages-N
• specific creation rules ?
  - [ ] cautious crawl + domains (when JS crawling finished cf JS Crawling via chrome headless #288)

see doc for details on option values https://github.com/medialab/hyphe/blob/master/doc/config.md