Move/add no-web-ui option to config

[AlexanderLanin]

Currently it must be specified on the command line, which is quite annoying when you want to use it consistently always.

This should also make parameters like username, password, twofa_seed optional (currently I need to specify them - as empty strings).

Alternatively no-web-ui can be implied by not having username, password and twofa_seed.

[lukpueh]

This is a good idea. Though, I'm in favor of being explicit and e.g. fail, if web should be used, but credentials are missing.

One gotcha we need to be aware of is the inverse --use-web flag on the list-advisories command. If there's a global config to use or not use web, it should affect that command too.

[netomi]

If you omit settings from the web ui in your default config, it will also not be used at all.

The default config basically tells otterdog what settings you are interested in maintaining through it, only settings present there will be explicitly queried and processed.