Databases: provide support for opml files, or sources json

[rumca-js]

It shall create a new sqlite table, and read sources from opml, sources json.

• Use remoteUrl to identify if it is json, or opml, or sqlite.
• There should be separate processors or handlers for database from json, opml file

Process

• currently file is downloaded and used as is, as a source
• we want to download sources json and insert sources
• we want to download entries json and insert entries