Add support for a base translation profile

[MLenterman]

Currently a translation profile only applies to a single zaaktype. As a result, common translation behavior and/or defaults need to be copied/pasted often for each zaaktype. We would like to have the option to add a base translation profile. With a base profile defined, the more specific per zaaktype profile should be merged on top of the base profile to obtain the final profile.

base profile	zaaktype profile	result profile
a: default value 1		a: default value 1
b: default value 2	b: 12345	b: 12345
c: default value 3		c: default value 3

The base profile can be just another profile in profiles.json with a '*' as zaakTypeIdentificatie.

• Add base translation profile
• Adapter that resolves the value for a given key from the merged base + specific translation profile
• Implement adapter everywhere where a translation profile property is used.
• User documentation