Add the possibility to enable crawler for feeds

locale/translations.go

miniflux-test

model/feed.go

reader/feed/handler.go

reader/processor/processor.go

reader/scraper/scraper.go

server/api/controller/feed.go

server/template/common.go

server/template/html/add_subscription.html

server/template/html/choose_subscription.html

server/template/html/edit_feed.html

server/template/views.go

server/ui/controller/entry.go

server/ui/controller/feed.go

server/ui/controller/subscription.go

server/ui/form/feed.go

server/ui/form/subscription.go

sql/schema_version_8.sql

sql/sql.go

storage/entry_query_builder.go