Use instance context instead of config context for saved data

[dmitriim]

This came out as part of discussion in #9

Currently when saving field data config context is being used. There may be some reasons for doing that, but this just indicates that something is missing in Moodle core APIs that pushes developers to workaround them and this should be fixed in core.

So rough tasks here would be:

• figure out core API limitations that forced to use config context
• if there are any, then raise MDL to fix those limitations
• when MDL is fixed (or if not needed), implement context using instance context instead of config context
• write upgrade script to fix all affected records
• release a new version of the plugin

@andrewhancox does this sound ok to you?